Compatibility with native gRPC client

[alexmt]

Proposal

Generate REST client that implements native gRPC client interface.

Motivation

The generated API client is great. However, it is slightly different from the native gRPC client interface.

• grpc-gateway client interface:

// GreeterGatewayClient is the interface for Greeter service client.
type GreeterGatewayClient interface {
	// Sends a greeting
	SayHello(context.Context, *HelloRequest) (*HelloReply, error)
}

• native grpc client

type GreeterClient interface {
	// Sends a greeting
	SayHello(context.Context, *HelloRequest,  ...grpc.CallOption) (*HelloReply, error)
}

If grpc-gateway client uses the same interface as native gRPC then it can be used as a drop-in replacement, making adoption much easier.