GrpcClient based on the ktor backend

[ShadowySpirits]

By integrating wire with ktor, we can create a multiplatform gRPC client using Kotlin that supports Linux, macOS, and Windows targets. It would be great if wire provided a Kotlin native implementation for the gRPC client.

Actually, I have created a demo project to accomplish this: ShadowySpirits/kotlin-native-grpc. And I am willing to contribute to upstream.

[oldergod]

GrpcClient is now open on all platforms #2677
and you can implement it yourself.

We don't have a clear plan related to a KMP http engine in Wire yet. We're also not sure we wanna commit into supporting one. I wanna see how OkHttp evolves!