Add SSE support to WebMvc.fn

This commit adds support for sending Server-Sent Events in WebMvc.fn,
through the ServerResponse.sse method that takes a SseBuilder DSL.
It also includes reference documentation.

Closes gh-25920

src/docs/asciidoc/web/webmvc-functional.adoc

@@ -229,6 +229,60 @@ Mono<ServerResponse> asyncResponse = webClient.get().retrieve().bodyToMono(Perso
 ServerResponse.async(asyncResponse);
 ----
 
+https://www.w3.org/TR/eventsource/[Server-Sent Events] can be provided via the
+static `sse` method on `ServerResponse`. The builder provided by that method
+allows you to send Strings, or other objects as JSON. For example:
+
+[source,java,indent=0,subs="verbatim,quotes",role="primary"]
+.Java
+----
+	public RouterFunction<ServerResponse> sse() {
+        return route(GET("/sse"), request -> ServerResponse.sse(sseBuilder -> {
+                	// Save the sseBuilder object somewhere..
+                }));
+	}
+
+	// In some other thread, sending a String
+	sseBuilder.send("Hello world");
+
+	// Or an object, which will be transformed into JSON
+    Person person = ...
+	sseBuilder.send(person);
+
+    // Customize the event by using the other methods
+    sseBuilder.id("42")
+            .event("sse event")
+            .data(person);
+
+	// and done at some point
+	sseBuilder.complete();
+----
+[source,kotlin,indent=0,subs="verbatim,quotes",role="secondary"]
+.Kotlin
+----
+    fun sse(): RouterFunction<ServerResponse> = router {
+        GET("/sse") { request -> ServerResponse.sse { sseBuilder ->
+            // Save the sseBuilder object somewhere..
+        }
+    }
+
+	// In some other thread, sending a String
+	sseBuilder.send("Hello world")
+
+	// Or an object, which will be transformed into JSON
+    val person = ...
+	sseBuilder.send(person)
+
+    // Customize the event by using the other methods
+    sseBuilder.id("42")
+            .event("sse event")
+            .data(person)
+
+	// and done at some point
+	sseBuilder.complete()
+----
+
+
 
 [[webmvc-fn-handler-classes]]
 === Handler Classes