Assign Admin role from OAuth2

[jplitza]

Describe the solution you'd like

A new setting akin to OAUTH2_ADDITIONAL_CLAIMS should enable accounts from OAuth2 sources to become admin users automatically (or have that role revoked).

Describe alternatives you've considered

Manually managing admin roles in pgAdmin in addition to the OAuth2 Identity Provider.

Additional context

This proposal might seem moot, since the admin role only provides the user management option, which might not seem relevant in OAuth2-only environments. However, when there are shared servers, one might want to delete the user that originally created them to transfer them to another user, even in a OAuth2-only environment (since pgAdmin cannot know when a user is no longer valid if they don't try to sign in).