Adds a package for testing time/timer/ticker functions. Implementation is limited to `NewTimer` and `NewContextTicker`, but will eventually be expanded to all `time` functions from the standard library as well as `context.WithTimeout()`, `context.WithDeadline()`.
Replaces `benbjohnson/clock` for the pubsub watchdog, as a proof of concept.
Eventually, as we expand functionality, we will replace most time-related functions with this library for testing.
adds a watchdog to our pubsub and runs it for Coder server.
If the watchdog times out, it triggers a graceful exit in `coder server` to give any provisioner jobs a chance to shut down.
c.f. #11950
