Prior to this commit, WebFlux had Protobuf codecs for managing the
`Message`/`"application/x-protobuf"` encoding and decoding.
The `com.google.protobuf:protobuf-java-util` library has additional
support for JSON (de)serialization, but this is not supported by
existing codecs.
This commit adds the new `ProtobufJsonEncode` and `ProtobufJsonDecoder`
classes that support this use case. Note, the `ProtobufJsonDecoder` has
a significant limitation: it cannot decode JSON arrays as
`Flux<Message>` because there is no available non-blocking parser able
to tokenize JSON arrays into streams of `Databuffer`. Instead,
applications should decode to `Mono<List<Message>>` which causes
additional buffering but is properly supported.
Closes gh-25457
17abb4d25d