Refactors the code that handles monitoring an agent websocket with pings and updating the connection times in the DB.
Consolidates v1 and v2 agent APIs under the same code for this.
One substantive change (not _just_ a refactor) is that I've made it so that we actually disconnect if the agent fails to respond to our pings, rather than the old behavior where we would update the database, but not actually tear down the websocket.
We're seeing some flaky tests related to agent connectivity - https://github.com/coder/coder/actions/runs/7286675441/job/19856270998
I'm pretty sure what happened in this one is that the client opened a connection while the wgengine was in the process of reconfiguring the wireguard device, so the fact that the peer became "active" as a result of traffic being sent was not noticed.
The test calls `AwaitReachable()` but this only tests the disco layer, so it doesn't wait for wireguard to come up.
I think we should be using TSMP for pinging and reachability, since this operates at the IP layer, and therefore requires that wireguard comes up before being successful.
This should also help with the problems we have seen where a TCP connection starts before wireguard is up and the initial round trip has to wait for the 5 second wireguard handshake retry.
fixes: #11294
Refactors our DRPC service definitions slightly.
In the previous version, I inserted the RPCs from the tailnet proto directly into the Agent service. This makes things hard to deal with because DRPC then generates a new set of methods with new interfaces with the `DRPCAgent_` prefixed. Since you can't have a single method that takes different argument types, we couldn't reuse the implementation of those RFCs without a lot of extra classes and pass-thru methods.
Instead, the "right" way to do it is to integrate at the DRPC layer. So, we have two DRPC services available over the Agent websocket, and register them both on the DRPC `mux`.
Since the tailnet proto RPC service is now for both clients and agents, I renamed some things to clarify and shorten.
This PR also removes the `TailnetAPI` implementation from the `agentapi` package, and the next PR in the stack replaces it with the implementation from the `tailnet` package.
* Add database tables for OAuth2 applications
These are applications that will be able to use OAuth2 to get an API key
from Coder.
* Add endpoints for managing OAuth2 applications
These let you add, update, and remove OAuth2 applications.
* Add frontend for managing OAuth2 applications
Bruno Quaresma