OAuth Support

[bpmct]

Today, our plugin only supports authentication via a providing a static token. We can add support for linking their Coder account via OAuth now that Coder has OAuth support.

• Today, admins configure Backstage as an OAuth application in https://coder.example.com/deployment/oauth2-provider/apps (we already have this)
  
  • Confirm that this works with Backstage once we add OAuth support into our plugin
• Admins should configure Coder as an auth provider in their app config file by providing the client id, client secret, and Coder URLs
• Users should be able to link their Coder accounts in Backstage via the Authentication Providers setting in their Account settings
  
  • We provide a sample component/wrapper to link their Coder accounts via OAuth instead of the token flow
• Admins do not need to change much in their plugin config, besides setting the auth method to OAuth and providing the OAuth app details. Existing components/APIs/wrappers should remain functionally the same.
• Write docs for how to configure Coder+Backstage+OAuth

[bpmct]

Prior art: https://github.com/coder/backstage.cdr.dev/commit/b61f77c4139433f11b8762db87f41378efeb2d3f

[f0ssel]

Closed with #151