Refactors Agent instance identity to be a SessionTokenProvider.
Refactors the CLI to create Agent clients via a centralized function, rather than add-hoc via individual command handlers and their flags.
This allows commands besides `coder agent`, but which still use the agent identity, to support instance identity authentication.
Fixes#19111 by unifying all API requests to go thru the SessionTokenProvider for auth credentials.
Refactors our use of `slogtest` to instantiate a "standard logger" across most of our tests. This standard logger incorporates https://github.com/coder/slog/pull/217 to also ignore database query canceled errors by default, which are a source of low-severity flakes.
Any test that has set non-default `slogtest.Options` is left alone. In particular, `coderdtest` defaults to ignoring all errors. We might consider revisiting that decision now that we have better tools to target the really common flaky Error logs on shutdown.
* Adds agenttest.New() helper function
* Makes sure agent gets closed on test cleanup
* Makes sure you don't forget to set session token
* Sets the agent and client logger automatically
Spike Curtis