grpc: Add Call Metadata (Headers and Trailers)

[Jozott00]

Subsystem
gRPC

Problem Description
Before this, we had no support for passing gRPC metadata between the client and the server.

Solution
This PR solves this on JVM and Native by providing a custom interface (GrpcMetadata).

[Mr3zee]

LGTM! Great work!
Couple of minor things below and a question:
Do you plan to add serializable versions of get/append/etc in a separate PR?

[Jozott00]

@Mr3zee I think I misunderstood the outcome of our last discussion about the serialization API for metadata. My understanding was, that we drop the API, as a full implementation would require differentiation between binary serialization and ascii serialization. So we leave it up to the user who can just call the upcoming codec() function to encode/decode the object.

But I am also fine with adding API for binary serialization only. In that case, should we call them get(key: Key) or getBinary(key: Key) to reflect that the serialization is binary?

[Mr3zee]

great work!