Closes #17982.

The purpose of this PR is to expose network latency via the API used by Coder Desktop.

This PR has the tunnel ping all known agents every 5 seconds, in order to produce an instance of:

message LastPing {
	// latency is the RTT of the ping to the agent.
	google.protobuf.Duration latency = 1;
	// did_p2p indicates whether the ping was sent P2P, or over DERP.
	bool did_p2p = 2;
	// preferred_derp is the human readable name of the preferred DERP region,
	// or the region used for the last ping, if it was sent over DERP.
	string preferred_derp = 3;
	// preferred_derp_latency is the last known latency to the preferred DERP
	// region. Unset if the region does not appear in the DERP map.
	optional google.protobuf.Duration preferred_derp_latency = 4;
}

The contents of this message are stored and included on all subsequent upsertions of the agent.
Note that we upsert existing agents every 5 seconds to update the last_handshake value.

On the desktop apps, this message will be used to produce a tooltip similar to that of the VS Code extension:

(wording not final)

Unlike the VS Code extension, we omit:

• The Latency of all available DERP regions. It seems not ideal to send a copy of this entire map for every online agent, and it certainly doesn't make sense for it to be on the Agent or LastPing message.
  If we do want to expose this info on Coder Desktop, we should consider how best to do so; maybe we want to include it on a more generic Netcheck message.
• The current throughput (Bytes up/down). This is out of scope of the linked issue, and is non-trivial to implement. I'm also not sure of the value given the frequency we're doing these status updates (every 5 seconds).
  If we want to expose it, it'll be in a separate PR.