Allow AbstractWebClientReactiveOAuth2AccessTokenResponseClient to be extended

Expected Behavior
Be able to extend the AbstractWebClientReactiveOAuth2AccessTokenResponseClient for custom AuthorizationGrantType implementations not just the four default ones implemented in the spring security framework.

Current Behavior

The current implementation of the oauth2 AbstractWebClientReactiveOAuth2AccessTokenResponseClient uses the type T extends AbstractOAuth2AuthorizationGrantRequest which implies the ability to extend the AbstractOAuth2AuthorizationGrantRequest and extend the AbstractWebClientReactiveOAuth2AccessTokenResponseClient to implement a custom authorization grant. The AbstractWebClientReactiveOAuth2AccessTokenResponseClient however has a package private constructor which restricts the ability to extend the mentioned class unless one puts the class in the org.springframework.security.oauth2.client.endpoint in their codebase.

Context
The OAuth2 spec allows for custom implementations of the OAuth2 grants by defining a grant type as specified in
OAuth2 RFC 6749 Section 4.5: Extension Grants.

Current implementation of the AuthorizationGrantType allows for custom grant types to be defined and the extension of AbstractOAuth2AuthorizationGrantRequest allows that as well.

However the inability to extend the AbstractWebClientReactiveOAuth2AccessTokenResponseClient leaves one with only one choice is to either duplicate the implementation in the afformentioned class, or write ones own implementation. Which is quite annoying when the base is already present in the framework code.

The request to allow for extension of this class has be done before i.e. https://github.com/spring-projects/spring-security/issues/10836 but with a failed mention to provide for customisation it was declined. However there was no mention or thought of custom grant type support.

It would be great to be able to create a custom extension of the said class in our own package structure rather than having to either reimplement the internals of the AbstractWebClientReactiveOAuth2AccessTokenResponseClient or place the new client into the org.springframework.security.oauth2.client.endpoint package.

Start with AbstractWebClientReactiveOAuth2AccessTokenResponseClient and AbstractOAuth2AuthorizationGrantRequest, then review how custom AuthorizationGrantType implementations are represented. Done means a subclass can be declared outside org.springframework.security.oauth2.client.endpoint and support a custom grant without duplicating the existing client implementation.