Support nested fields, path segments with merged master and working ci

[marsianin]

Closes https://github.com/grpc-ecosystem/protoc-gen-grpc-gateway-ts/issues/28

Adds support for 2 related cases in URLs:

Support path segments like in https://google.aip.dev/127, where the URL contains a pattern like post: "/v1/{parent=publishers/*}/books" Support nested field names in the URL, where the URL is structured like:

  option (google.api.http) = {
    patch: "/v3/{intent.name=projects/*/locations/*/agents/*/intents/*}"
    body: "intent"
  };

This gets translated to /v3/${req["intent"]["name"]} While here, use the newer protoc-gen-go-grpc plugin for generating server test code, due to deprecated usage of plugin=grpc (see https://github.com/protocolbuffers/protobuf-go/releases/tag/v1.20.0#v1.20-grpc-support)