Kotlin/JS support for GrpcClient

[tianyu]

Currently, there is no implementation of com.squareup.wire.GrpcClient for Kotlin/JS, which is a shame because everything else works.

It's difficult for JS to speak gRPC natively since web browser APIs don't support bidirectional streaming. But maybe we can include an implementation of GrpcClient that speaks gRPC WEB.

Here's a gist of how that could work.

[oldergod]

GrpcClient is an abstract class and you should be able to create your implementation of it for the JS platform.