chore: check server version on sign-in and launch

[deansheather]

Updates CredentialModel to have an additional state Unknown during startup while credentials are validated in the background asynchronously (15s timeout).

While loading credentials on startup, the tray app shows a loading message.

While updating credentials, we now check that the server version falls in our expected range.

The sign in page now uses a WinUI 3 Dialog to show errors.

Closes #42

[deansheather]

@spikecurtis I pretty heavily reworked CredentialManager:

• Duplicate LoadCredentials calls now get merged (the same Task will be returned from both, so they will receive identical exceptions/results)
• A SetCredentials call during an ongoing LoadCredentials call will cancel it
• Added tests
  • To add tests I needed to add ICoderApiClientFactory and ICoderApiClient for mocking purposes
  • Pulled the NativeApi components of CredentialManager into a new type WindowsCredentialBackend (and added interface ICredentialBackend for mocking)

[deansheather]

To protect things from concurrent access:

• SetCredentials holds _opLock for the entire operation
• LoadCredentials will hold _opLock while it creates the "inner" task then releases it
  • The "inner" task will grab the lock again when it's ready to write the loaded value

[spikecurtis]

I like the new CredentialManager overall, but still a bit concerned about invoking callbacks while holding the lock. A couple other small nits FYI.

[spikecurtis]

Just to check my understanding here, because we are not awaiting the task, it doesn't matter if the task throws some exception, like being canceled. And, we don't block for it to complete, right?

I ask because we want the rest of this function to still run and handle closing of the tray window.

[deansheather]

Yep, we run it in the background and ignore all errors. If the credentials fail to load, the state will transition to "invalid" which will show the "please sign in" screen.