Consider adding ClientRegistrationIdResolver to ExchangeFilterFunctions

[sjohnr]

We should align (Server|Servlet)OAuth2AuthorizedClientExchangeFilterFunction with OAuth2ClientHttpRequestInterceptor which introduces a ClientRegistrationIdResolver as a flexible strategy for resolving the clientRegistrationId for a given request. For WebClient-based ExchangeFilterFunctions, the interface could be:

@FunctionalInterface
public interface ClientRegistrationIdResolver {

	@Nullable
	String resolve(ClientRequest request);

}