Add ServerOAuth2AuthorizedClientExchangeFilterFunction

[rwinch]

Summary

We should make it easy to use an OAuth2AuthorizedClient with WebClient. This can be done by providing an API that looks like this:

@Bean
WebClient webClient() {
	return WebClient.builder()
			.filter(new ServerOAuth2AuthorizedClientExchangeFilterFunction())
			.build();
}

import static org.springframework.security.oauth2.client.web.reactive.function.client.ServerOAuth2AuthorizedClientExchangeFilterFunction.oauth2AuthorizedClient;
// ...
@GetMapping("/oauth/api")
public String oauthApi(@RegisteredOAuth2AuthorizedClient OAuth2AuthorizedClient authorizedClient) {
	String response = this.webClient
		.get()
		.uri(userInfoEndpointUri)
		.attributes(oauth2AuthorizedClient(authorizedClient))
		.retrieve()
		.bodyToMono(String.class)
		.block();
	// ...
}

At the moment the only thing that is done is adding. However, once refresh tokens are supported we will add automatically refreshing the token as well.