Add option including JSON default value for transcoding

[mutexd]

Motivation:

I'm using grpc-transcoding so that I can have one grpc-service implementation which can serve both Grpc/REST entry point. However, the JsonPrinter's default behavior excludes default values such as int32:0, string:"". Therefore, a GrpcResponse with
{ "message": "Hello", "status": 0 }
will become like this for REST response.
{ "message": "Hello" }
Even though it's valid in terms of ProtoJSON, it would be nice to have default values because JSON in REST conventionally includes default value like this.

Draft for #261 (need discussion)
Adds a flag for vertx-grpc-protoc-plugin2 and directly embeds printer.includingDefaultValueFields() into transcoding's encoder method. Changing the ServiceMethod signature should be avoided and therefore I approach the issue from vertx-grpc-protoc-plugin2.

[mutexd]

If the approach taken doesn't align with the structure of the project, I would like to delete this PR and create an issue. A feedback would be appreciated.

[zZHorizonZz]

Hello, thank you for the contribution.

I think it would be better to convert this to a draft and open an issue so it can be discussed, more in depth.