Adds a new statsReporter subcomponent of the agent, which in a later PR will be used to report stats over the v2 API.
Refactors the logic a bit so that we can handle starting and stopping stats reporting if the agent API connection drops and reconnects.
* fix: Refactor agent to consume API client
This simplifies a lot of code by creating an interface for
the codersdk client into the agent. It also moves agent
authentication code so instance identity will work between
restarts.
Fixes#3485 and #4082.
* Fix client reconnections
Spike Curtis
Colin Adler
Kyle Carberry
Garrett Delfosse
Ammar Bandukwala