[dts] Generate PromiseClient type definitions in d.ts file

[at-ishikawa]

Issue

In the _grpc_web_pb.js file, PromiseClient is defined and some of its methods return the promise, instead of ClientReadableStream.
But there is no type definition for PromiseClient in d.ts file, so we can't use this class without warnings of type checking.

Proposal

Generate PromiseClient type definitions in d.ts file on top of the existing Client definition.
The methods of this PromiseClient have Promise return type only if the RPC method is not client streaming nor server streaming RPC.

Note that I did not change signature for the method which returns server streaming, because the generated js file does not return Promise, but not sure this is correct.
Please tell me if I need to change something.

Sample

The generated d.ts file of net/grpc/gateway/examples/helloworld/helloworld.proto is like following.

$ cat helloworld_grpc_web_pb.d.ts
import * as grpcWeb from 'grpc-web';
import {
  HelloReply,
  HelloRequest,
  RepeatHelloRequest} from './helloworld_pb';

export class GreeterClient {
  constructor (hostname: string,
               credentials: null | { [index: string]: string; },
               options: null | { [index: string]: string; });

  sayHello(
    request: HelloRequest,
    metadata: grpcWeb.Metadata,
    callback: (err: grpcWeb.Error,
               response: HelloReply) => void
  ): grpcWeb.ClientReadableStream<HelloReply>;

  sayRepeatHello(
    request: RepeatHelloRequest,
    metadata: grpcWeb.Metadata
  ): grpcWeb.ClientReadableStream<HelloReply>;

  sayHelloAfterDelay(
    request: HelloRequest,
    metadata: grpcWeb.Metadata,
    callback: (err: grpcWeb.Error,
               response: HelloReply) => void
  ): grpcWeb.ClientReadableStream<HelloReply>;

}

export class GreeterPromiseClient {
  constructor (hostname: string,
               credentials: null | { [index: string]: string; },
               options: null | { [index: string]: string; });

  sayHello(
    request: HelloRequest,
    metadata: grpcWeb.Metadata
  ): Promise<HelloReply>;

  sayRepeatHello(
    request: RepeatHelloRequest,
    metadata: grpcWeb.Metadata
  ): grpcWeb.ClientReadableStream<HelloReply>;

  sayHelloAfterDelay(
    request: HelloRequest,
    metadata: grpcWeb.Metadata
  ): Promise<HelloReply>;

}

[stanley-cheung]

Thanks for your contributions!