feat: http transcoding grpc

[WayneWang12]

References #1461.

Most code of this commit are from file https://github.com/cloudstateio/cloudstate/blob/master/proxy/core/src/main/scala/io/cloudstate/proxy/HttpApi.scala

Just do some change to make it may work in akka-grpc.

[lightbend-cla-validator]

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[WayneWang12]

I've only implement scala mode. I'm not quite familiar with Java.

Also, I think we can make grpc client works for json format http request. That will make akka-grpc usable even we only use protobuf to describe service protocols.

[lightbend-cla-validator]

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[johanandren]

Closing this since continued in #1834