Closes#14729
Expands the Coordination controller used by the CLI client to allow multiple tunnel destinations (agents). Our current client uses just one, but this unifies the logic so that when we add Coder VPN, 1 is just a special case of "many."
chore of #14729
Refactors the `ServerTailnet` to use `tailnet.Controller` so that we reuse logic around reconnection and handling control messages, instead of reimplementing. This unifies our "client" use of the tailscale API across CLI, coderd, and wsproxy.
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
Spike Curtis