Closes #16347 (partial - adds mqtt.max_connections).

What this PR does

Adds mqtt.max_connections, a per-node connection limit for the MQTT plugin. When the limit is reached, the broker sends a CONNACK with return code not_authorized (MQTT 3.x/4) or reason code quota_exceeded (MQTT 5) and closes the connection.

How it works

The limit is checked in rabbit_mqtt_processor:process_connect/5 as the first step in the CONNECT packet handler, before authentication. It does not operate at the Ranch transport layer, so the TCP listen queue is unaffected and Ranch continues accepting connections normally.

The active connection count uses ets:info(persistent_term:get(?PG_SCOPE), size). The MQTT plugin creates a node-local PG scope in rabbit_mqtt_sup; each connection joins it via pg:join/3 in register_client_id/4, using {VHost, ClientId} as the group key. Since the MQTT spec requires unique client IDs per vhost, which RabbitMQ enforces, each group has exactly one member. The PG scope ETS table therefore has one row per active connection node-wide, covering all listeners and transports (plain TCP, TLS, and Web MQTT). ets:info/2 reads this count in O(1) - important given that MQTT nodes can host a very large number of connections.

The check runs before register_client_id/4, so the current connection is not yet counted; the limit comparison is >= max_connections.

Configuration

mqtt.max_connections = 1000

The default is infinity (no limit). The setting accepts a non-negative integer or infinity.

Testing

Adds node_connection_limit to the limit group in auth_SUITE. The test sets the limit to 0 via RPC so the first CONNECT attempt is rejected, covering both MQTT v4 and v5 via the existing expected_connection_limit_error/1 helper.