Added authorizationCodeGrant method to OAuthAuthenticator

[cwurtz]

This PR adds a method to exchange an authorization code for a Bearer token containing the access_token & id_token.

closes: #284

[luisrudge]

Thanks for the PR! Do you think it makes more sense to call it authorizationCodeGrant since that's how we document it?

[cwurtz]

I made it authorizeCodeGrant to denote the action, but looking at the docs, "Authorization Code" is more of an adjective describing the grant type so it probably doesn't make sense for me to have named it how I did. authorizationCodeGrant sounds like a better option to me as well

[cwurtz]

Updated the PR to change authorizeCodeGrant -> authorizationCodeGrant