Add @LocalRSocketServerPort support

Add an appication context initializer to detect and store the
active RSocket port in the Environment under
`local.rsocket.server.port`.

Additionally add a `@LocalServerPort` that provides a convenient
alternative to `@Value`.

See gh-18287

Co-authored-by: Eddú Meléndez <eddu.melendez@gmail.com>

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/rsocket/RSocketServerAutoConfigurationTests.java

@@ -24,6 +24,7 @@ import org.springframework.boot.rsocket.server.RSocketServerFactory;
 import org.springframework.boot.rsocket.server.ServerRSocketFactoryCustomizer;
 import org.springframework.boot.test.context.runner.ApplicationContextRunner;
 import org.springframework.boot.test.context.runner.ReactiveWebApplicationContextRunner;
+import org.springframework.boot.web.context.RSocketPortInfoApplicationContextInitializer;
 import org.springframework.boot.web.server.WebServerFactoryCustomizer;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
@@ -39,6 +40,7 @@ import static org.mockito.Mockito.mock;
  * Tests for {@link RSocketServerAutoConfiguration}.
  *
  * @author Brian Clozel
+ * @author Verónica Vásquez
  */
 class RSocketServerAutoConfigurationTests {
 
@@ -80,6 +82,17 @@ class RSocketServerAutoConfigurationTests {
 						.hasSingleBean(ServerRSocketFactoryCustomizer.class));
 	}
 
+	@Test
+	void shouldSetLocalServerPortWhenRSocketServerPortIsSet() {
+		reactiveWebContextRunner().withPropertyValues("spring.rsocket.server.port=0")
+				.withInitializer(new RSocketPortInfoApplicationContextInitializer()).run((context) -> {
+					assertThat(context).hasSingleBean(RSocketServerFactory.class)
+							.hasSingleBean(RSocketServerBootstrap.class)
+							.hasSingleBean(ServerRSocketFactoryCustomizer.class);
+					assertThat(context.getEnvironment().getProperty("local.rsocket.server.port")).isNotNull();
+				});
+	}
+
 	@Test
 	void shouldUseCustomServerBootstrap() {
 		contextRunner().withUserConfiguration(CustomServerBootstrapConfig.class).run((context) -> assertThat(context)