Extend support for oauth: add token acquisition

[amattioc]

@amattioc this works for IMF_RESTR. To extend this to support token acquisition we would need to include client_id, authority, and scope in the provider definition, so users don't have to enter that information all the time. What I do not know is how you think this should be set up. We could introduce a new option to AuthenticationMethods "OAuth2", We could then create an OAuth2Config object and have that object as an input to Provider. If this sounds ok to you i can do it and set up the fetching of the token.

I guess we also need to expose the AuthenticationMethod field to the add provider UI.

Originally posted by @aboddie in #278