Adds some debug endpoints for looking into the DERP server.

The `api/v2/debug/derp/traffic` endpoint requires the `ss` utility to be present in order to function.  I have *not* added the `iproute2` package to our base image as it adds 11MB, so this endpoint won't be useful by default.  However, in a debugging situation, we could exec into the container and then `apk add iproute2`, or build a special debug image.

The `api/v2/debug/expvar` handler contains DERP metrics as well as commandline and memstats.

Example:

```
{
"alert_failed": 0,
"alert_generated": 0,
"cmdline": ["/Users/spike/repos/coder/build/coder_darwin_arm64","--global-config","/Users/spike/repos/coder/.coderv2","server","--http-address","0.0.0.0:3000","--swagger-enable","--access-url","http://127.0.0.1:3000","--dangerous-allow-cors-requests=true"],
"derp": {"accepts": 1, "average_queue_duration_ms": 0, "bytes_received": 0, "bytes_sent": 0, "counter_packets_dropped_reason": {"gone_disconnected": 0, "gone_not_here": 0, "queue_head": 0, "queue_tail": 0, "unknown_dest": 0, "unknown_dest_on_fwd": 0, "write_error": 0}, "counter_packets_dropped_type": {"disco": 0, "other": 0}, "counter_packets_received_kind": {"disco": 0, "other": 0}, "counter_tcp_rtt": {}, "counter_total_dup_client_conns": 0, "gauge_clients_local": 1, "gauge_clients_remote": 0, "gauge_clients_total": 1, "gauge_current_connections": 1, "gauge_current_dup_client_conns": 0, "gauge_current_dup_client_keys": 0, "gauge_current_file_descriptors": 0, "gauge_current_home_connections": 1, "gauge_memstats_sys0": 20874504, "gauge_watchers": 0, "got_ping": 0, "home_moves_in": 0, "home_moves_out": 0, "multiforwarder_created": 0, "multiforwarder_deleted": 0, "packet_forwarder_delete_other_value": 0, "packets_dropped": 0, "packets_forwarded_in": 0, "packets_forwarded_out": 0, "packets_received": 0, "packets_sent": 0, "peer_gone_disconnected_frames": 0, "peer_gone_not_here_frames": 0, "sent_pong": 0, "unknown_frames": 0, "version": "1.47.0-dev20240214-t64db8c604"},
"memstats": {"Alloc":286506256,"TotalAlloc":297594632,"Sys":310621512,"Lookups":0,"Mallocs":304204,"Frees":171570,"HeapAlloc":286506256,"HeapSys":294060032,"HeapIdle":3694592,"HeapInuse":290365440,"HeapReleased":3620864,"HeapObjects":132634,"StackInuse":3735552,"StackSys":3735552,"MSpanInuse":347256,"MSpanSys":358512,"MCacheInuse":9600,"MCacheSys":15600,"BuckHashSys":1469877,"GCSys":9434896,"OtherSys":1547043,"NextGC":551867656,"LastGC":1707892877408883000,"PauseTotalNs":1247000,"PauseNs":[200333,229375,239875,209542,106958,203792,57125,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0],"PauseEnd":[1707892876217481000,1707892876219726000,1707892876222273000,1707892876226151000,1707892876234815000,1707892877398146000,1707892877408883000,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0],"NumGC":7,"NumForcedGC":0,"GCCPUFraction":0.0022425810335762954,"EnableGC":true,"DebugGC":false,"BySize":[{"Size":0,"Mallocs":0,"Frees":0},{"Size":8,"Mallocs":14396,"Frees":9143},{"Size":16,"Mallocs":89090,"Frees":50507},{"Size":24,"Mallocs":40839,"Frees":24456},{"Size":32,"Mallocs":22404,"Frees":12379},{"Size":48,"Mallocs":51174,"Frees":23718},{"Size":64,"Mallocs":15406,"Frees":3501},{"Size":80,"Mallocs":6688,"Frees":2352},{"Size":96,"Mallocs":2567,"Frees":374},{"Size":112,"Mallocs":19371,"Frees":16883},{"Size":128,"Mallocs":2873,"Frees":1061},{"Size":144,"Mallocs":5600,"Frees":2742},{"Size":160,"Mallocs":2159,"Frees":622},{"Size":176,"Mallocs":454,"Frees":86},{"Size":192,"Mallocs":227,"Frees":128},{"Size":208,"Mallocs":1407,"Frees":732},{"Size":224,"Mallocs":1365,"Frees":1090},{"Size":240,"Mallocs":82,"Frees":48},{"Size":256,"Mallocs":310,"Frees":162},{"Size":288,"Mallocs":1945,"Frees":562},{"Size":320,"Mallocs":1200,"Frees":458},{"Size":352,"Mallocs":133,"Frees":33},{"Size":384,"Mallocs":582,"Frees":51},{"Size":416,"Mallocs":747,"Frees":200},{"Size":448,"Mallocs":113,"Frees":22},{"Size":480,"Mallocs":34,"Frees":21},{"Size":512,"Mallocs":951,"Frees":91},{"Size":576,"Mallocs":364,"Frees":122},{"Size":640,"Mallocs":532,"Frees":270},{"Size":704,"Mallocs":93,"Frees":39},{"Size":768,"Mallocs":83,"Frees":35},{"Size":896,"Mallocs":308,"Frees":175},{"Size":1024,"Mallocs":226,"Frees":122},{"Size":1152,"Mallocs":198,"Frees":100},{"Size":1280,"Mallocs":314,"Frees":171},{"Size":1408,"Mallocs":77,"Frees":47},{"Size":1536,"Mallocs":80,"Frees":54},{"Size":1792,"Mallocs":199,"Frees":107},{"Size":2048,"Mallocs":112,"Frees":48},{"Size":2304,"Mallocs":71,"Frees":32},{"Size":2688,"Mallocs":206,"Frees":81},{"Size":3072,"Mallocs":39,"Frees":15},{"Size":3200,"Mallocs":16,"Frees":7},{"Size":3456,"Mallocs":44,"Frees":29},{"Size":4096,"Mallocs":192,"Frees":83},{"Size":4864,"Mallocs":44,"Frees":25},{"Size":5376,"Mallocs":105,"Frees":43},{"Size":6144,"Mallocs":25,"Frees":5},{"Size":6528,"Mallocs":22,"Frees":7},{"Size":6784,"Mallocs":3,"Frees":0},{"Size":6912,"Mallocs":4,"Frees":2},{"Size":8192,"Mallocs":59,"Frees":10},{"Size":9472,"Mallocs":31,"Frees":12},{"Size":9728,"Mallocs":5,"Frees":2},{"Size":10240,"Mallocs":5,"Frees":0},{"Size":10880,"Mallocs":27,"Frees":11},{"Size":12288,"Mallocs":4,"Frees":1},{"Size":13568,"Mallocs":4,"Frees":2},{"Size":14336,"Mallocs":9,"Frees":2},{"Size":16384,"Mallocs":10,"Frees":2},{"Size":18432,"Mallocs":4,"Frees":2}]},
"warning_failed": 0,
"warning_generated": 0
}
```

If we find the DERP metrics useful we could consider how to include them in Prometheus scrapes based on the tailnet `varz` package.  That's for a later PR if at all.