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

[marsianin]

Closes #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)

[adolfheir]

can merge？

[marsianin]

it looks like more than six month no activities in this repo related to review or merging

[uthark]

any updates on it?