DefaultOAuth2AuthorizedClientManager and DefaultServerOAuth2AuthorizedClientManager Alignment

[jzheaux]

DefaultOAuth2AuthorizedClientManager maps the username and password fields for the OAuth 2.0 password grant.

However, DefaultServerOAuth2AuthorizedClientManager does not do this.

The rationale given for excluding is:

I don't think it's a good idea to read the request body for each request mapping by default, so I removed that code. The default behaviour for DefaultContextAttributesMapper should read supported query parameters only.

Given the above reasoning, the functionality for DefaultOAuth2AuthorizedClientManager should also be removed.