Support binding gRPC services that are defined as object literals.

[igalshilman]

This PR adds an additional way of binding gRPC services.

Here are few examples:

const greeter = {
  greet: (req) => TestResponse.create({ greeting: `Hello` })
};

or the fully typed variant:

const greeter: GreeterService = {
  greet:  async (req: GreetRequest): Promise<GreetResponse> => {
    return TestResponse.create({ greeting: `Hello` });
  },
};