feat: support of grpc.ClientConnInterface allows custom streams

[DenKoren]

This makes StreamDirector function to accept grpc.ClientConnInterface instead of strict *grpc.ClientConn implementation. This allows to use custom connection implementations and stubs, like inprocess stream channeling without real network connection (https://pkg.go.dev/github.com/fullstorydev/grpchan/inprocgrpc)

I added the changed version interfaces to make this changes backwards-compatible. Maybe it is woth of releasing another version of grpc-proxy package, that is not compatible with previous one on interface level.

If we just change StreamDirector declaration, all implementations of StreamDirector will become incompatible with the new interface :(