Add OAuth2TokenResponseEnhancer to enhance the access token response

[Hccake]

Closes gh-925

[jgrandja]

@Hccake As mentioned in this comment:

I feel the duplication of initializing the defaults for OAuth2AccessTokenResponse.Builder is quite minimal and don't think there is much value for reuse at this point.

The proposed OAuth2TokenResponseEnhancer is a fine-grained interface and I'm not sure how much use it will receive at this point. Although AuthenticationSuccessHandler is more coarse-grained, it provides greater flexibility in customizing the access token response. I still feel the code duplication for initializing the OAuth2AccessTokenResponse.Builder in the consuming application is acceptable. Please keep in mind that this project is a framework and code customization is expected in the consuming application. We do our best to allow for code reuse but it's also expected that the consuming application will customize on their end. If the code duplication is quite an effort then we will look at improving. But in this case, the code duplication is quite minimal and adding a new interface does not make sense at this point.

Either way, I will leave gh-925 open and will monitor the comments to see if further improvements are needed.