refactors `tailnetAPIConnector` to use the `Dialer` interface in `tailnet`, introduced lower in this stack of PRs. This will let us use the same Tailnet API handling code across different things that connect to the Tailnet API (CLI client, coderd, workspace proxies, and soon: Coder VPN).
chore re: #14729
Refactors the way clients of the Tailnet API (clients of the API, which include both workspace "agents" and "clients") interact with the API. Introduces the idea of abstract "controllers" for each of the RPCs in the API, and implements a Coordination controller by refactoring from `workspacesdk`.
chore re: #14729
Fixes#12560
When gracefully disconnecting from the coordinator, we would send the Disconnect message and then close the dRPC stream. However, closing the dRPC stream can cause the server not to process the Disconnect message, since we use the stream context in a `select` while sending it to the coordinator.
This is a product bug uncovered by the flake, and probably results in us failing graceful disconnect some minority of the time.
Instead, the `remoteCoordination` (and `inMemoryCoordination` for consistency) should send the Disconnect message and then wait for the coordinator to hang up (on some graceful disconnect timer, in the form of a context).
When an agent receives a node, it responds with an ACK which is relayed
to the client. After the client receives the ACK, it's allowed to begin
pinging.
Currently, importing `codersdk` just to interact with the API requires
importing tailscale, which causes builds to fail unless manually using
our fork.
