Check if the GitHub oAuth is available when asking to sign in the Pull Requests plugin

[vinokurig]

Is your enhancement related to a problem? Please describe.

If GitHub identity provider is not setup or GitHub oAuth is not configured, GitHub sign in functionality for the vscode GitHub Pull Request plugin won't work.

Describe the solution you'd like

Check if GitHub oAuth is available. If not, do not show the notification to sign in the GitHub plugin:

Describe alternatives you've considered

Additional context

[benoitf]

Hello, we discussed at the devex call and we wanted to do the following:

two steps like short term and mid-long/term:

• : Short term: if the github identity provider is not setup, we should notify the user on how to configure it by referencing the documentation (if one project is coming from github)
• : Mid term: Add a redirect to keycloak URL and how to do the setup (documentation)

[vinokurig]

Is blocked by #18791