Fixes https://github.com/coder/internal/issues/933
Refactors CLI tests that check the `--auth` flag parsing for various public clouds into a unit test that just creates the agent Client and asserts on the type.
Testing that the agent client actually authenticates correctly with these auth types is well covered by Coderd tests, so we don't need to retread that ground here, and the deleted tests were flaky on Windows.
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.
Spike Curtis