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
36a07aa897