Introduce EmbeddedWebServerInitializedEvent

This commit adds `EmbeddedWebServerInitializedEvent`, which holds the
`EmbeddedWebServer` information when the application context starts.
The `EmbeddedServletContainerInitializedEvent` now inherits from that
type and holds additional information, the
`EmbeddedWebApplicationContext`.

Closes gh-8208

spring-boot-actuator/src/test/java/org/springframework/boot/actuate/autoconfigure/EndpointWebMvcAutoConfigurationTests.java

@@ -896,7 +896,7 @@ public class EndpointWebMvcAutoConfigurationTests {
 		@Override
 		public void onApplicationEvent(EmbeddedServletContainerInitializedEvent event) {
 			if (event.getApplicationContext() != this.rootContext) {
-				this.servletContainer = event.getEmbeddedServletContainer();
+				this.servletContainer = event.getEmbeddedWebServer();
 			}
 		}