Support Protobuf serialization in WebFlux

This commit introduces Protobuf support in WebFlux via dedicated
codecs.

Flux<Message> are serialized/deserialized using delimited Protobuf
messages with the size of each message specified before the message
itself. In that case, a "delimited=true" parameter is added to the
content type.

Mono<Message> are expected to use regular Protobuf message
format (without the size prepended before the message).

Related HttpMessageReader/Writer are automatically registered when the
"com.google.protobuf:protobuf-java" library is detected in the classpath,
and can be customized easily if needed via CodecConfigurer, for example
to specify protocol extensions via the ExtensionRegistry based
constructors.

Both "application/x-protobuf" and "application/octet-stream" mime types
are supported.

Issue: SPR-15776

spring-webflux/src/test/proto/sample.proto

@@ -0,0 +1,12 @@
+option java_package = "org.springframework.web.reactive.protobuf";
+option java_outer_classname = "OuterSample";
+option java_multiple_files = true;
+
+message Msg {
+    optional string foo = 1;
+    optional SecondMsg blah = 2;
+}
+
+message SecondMsg {
+    optional int32 blah = 1;
+}