Discover Auth Parameters During Login

[tri-adam]

Currently, there are a number of parameters that are hard-coded in loginCmd:

https://github.com/sylabs/compute-cli/blob/f7deb67f2584382b64e024d46a370284b352242e/internal/app/cli/login.go#L19-L47

What we probably want to happen is that the user/administrator can configure a single URL, and the rest of these parameters are discovered from a well-known endpoint based on that URL. The OAuth 2.0 Authorization Server Metadata spec covers some, but not all of these parameters, so won't be suitable on its own. The back end should host a discovery endpoint that will act as a source for this info.

The parameters that will need to be discovered are (at this moment):

• OAuth ClientID
• oauth2.Endpoint, which itself contains the AuthURL and TokenURL values. These can/should be discovered from a /.well-known/oauth-authorization-server endpoint.
• The listenAddr and authPath in the auth package