Add AuthoritiesMapper setter for reactive OAuth2Login

Allow the configuration of a custom GrantedAuthorityMapper for reactive OAuth2Login

- Add setter in OidcAuthorizationCodeReactiveAuthenticationManager
  and OAuth2LoginReactiveAuthenticationManager

- Use an available GrantedAuthorityMapper bean to configure the default ReactiveAuthenticationManager

Fixes gh-8324

docs/manual/src/docs/asciidoc/_includes/reactive/oauth2/login.adoc

@@ -160,3 +160,21 @@ SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
 	return http.build();
 }
 ----
+
+You may register a `GrantedAuthoritiesMapper` `@Bean` to have it automatically applied to the default configuration, as shown in the following example:
+
+[source,java]
+----
+@Bean
+public GrantedAuthoritiesMapper userAuthoritiesMapper() {
+    ...
+}
+
+@Bean
+SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
+	http
+		// ...
+		.oauth2Login(withDefaults());
+	return http.build();
+}
+----