Fluent API for explicit media readers and writers

[romain-grecourt]

Example:

Explicitly use the JSON-P writer to send a JsonObject instance as a server response payload:

res.send(JsonpSupport.writer().marshall(Json.createObjectBuilder().add("foo", "bar").build()));

Use the JSON-P reader to explicitly consume a server request payload:

JsonpSupport.reader().unmarshall(req.content()).thenAccept(System.out::println);

This can be implemented with default methods on MessageBodyReader and MessageBodyWriter:

MessageBodyReader:

default Single<T> unmarshall(MessageBodyReadableContent content) {
    return content.readerContext().unmarshall(content, this);
}

MessageBodyWriter:

default Function<MessageBodyWriterContext, Publisher<DataChunk>> marshall(T value) {
    return ctx -> ctx.marshall(Single.just(value), this);
}

The marshall method added on MessageBodyWriter requires a new variant of ServerResponse.send:

<T> Single<ServerResponse> send(Function<MessageBodyWriterContext, Publisher<DataChunk> fn);

It would also require a new variant of WebClientRequestBuilder.submit:

Single<WebClientResponse> submit(Function<MessageBodyWriterContext, Publisher<DataChunk> fn);

These changes are now feasible with #1905, it provides static accessors for reader and writer instances and replaces the old marshall / unmarshall variants on the reader and writer context that take an operator class with an operator instance.

Note that this change does not break any existing API, so it can be added later.
It seems that it should also be the default way the Helidon built-in service deal with media types.

[romain-grecourt]

@tomas-langer FYI