grpc web library generation

[Pritesh-Patel]

Generate grpc-web artifacts for the frontend to consume - the proxy can be added at a later stage

What options do we need to support?

These are from the grpc-web repo:

Import Style
import_style=closure: The default generated code has Closure goog.require() import style.

import_style=commonjs: The CommonJS style require() is also supported.

import_style=commonjs+dts: (Experimental) In addition to above, a .d.ts typings file will also be generated for the protobuf messages and service stub.

import_style=typescript: (Experimental) The service stub will be generated in TypeScript.

Note: commonjs+dts and typescript only works with --grpc-web_out= import style.

Wire Format Mode
For more information about the gRPC-Web wire format, please see the specification here.

mode=grpcwebtext: The default generated code sends the payload in the grpc-web-text format.

Content-type: application/grpc-web-text
Payload are base64-encoded.
Both unary and server streaming calls are supported.
mode=grpcweb: A binary protobuf format is also supported.

Content-type: application/grpc-web+proto
Payload are in the binary protobuf format.
Only unary calls are supported for now.

[Pritesh-Patel]

closed with #7