re: #14730

Adds a protocol controller for WorkspaceUpdates RPC that takes all the agents we learn about over the RPC, and programs them into the Coordination controller, so that we set up tunnels to all the agents.

Handling DNS is in a PR up the stack, as is actually wiring it up to anything.

re: #14730

Adds support for the workspace updates protocol controller to also program DNS names for each agent.

Right now, we only program names like `myagent.myworkspace.me.coder` and `myworkspace.coder.` (if there is exactly one agent in the workspace).  We also want to support `myagent.myworkspace.username.coder.`, but for that we need to update WorkspaceUpdates RPC to also send the workspace owner's username, which will be in a separate PR.

re: #14730

Adds support in `tailnet.Controller` for WorkspaceUpdates.

Also checks configured controllers against the clients returned by the dialer, so that if we connect with a dialer that doesn't support an RPC (for instance the in-memory dialer for ServerTailnet doesn't support WorkspaceUpdates), we throw an error if there is a controller expecting it.

closes #14730

Adds support for WorkspaceUpdates to the WebsocketDialer. This allows us to dial the new endpoint added in #14847 and connect it up to a `tailnet.Controllers` to connect to all agents over the tailnet.

I refactored the fakeWorkspaceUpdatesProvider to a mock and moved it to `tailnettest` so it could be more easily reused.  The Mock is a little more full-featured.