Support nested fields, path segments

[mbarrien]

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)

[hcwang512]

This fix works perfect for our use case, please merge it. @mbarrien

[yeluyang]

we really need this patch, can someone do some magic and make ci happy ? 😭