akka-http-oauth2-provider miss a way to extend the AccessToken response

[Christophe Pache]

Would be great, just as in scalaoauth2.provider.OAuth2Provider from play2-oauth2-provider, to be able to extend the response with other data.

Just as the scalaoauth2.provider.OAuth2Provider.responseAccessToken. It might also just use the default json formatter but it would be great to be able to override it.

[Tsuyoshi Yoshizawa]

Thanks for the PR.