QFE should optionally expose httpgrpc on through gRPC

[GiedriusS]

Is your proposal related to a problem?

I want to use Cortex Ruler with Thanos (and bring these two projects closer together) but Cortex Ruler uses https://github.com/weaveworks/common/tree/master/httpgrpc to send requests. Since the Cortex project prefers using gRPC, let's implement this interface in Thanos QFE.

Describe the solution you'd like

Adding an optional gRPC server to QFE that exposes this interface.

Describe alternatives you've considered

N/A

Additional context

N/A

[cswpy]

I can take this issue

[cswpy]

As I am new to the repo, just want to double-check the steps to host a gRPC server in QFE:

1. Add a grpcConfig to queryFrontendConfig and a flag to query-frontend parser.
2. Add weaveworks/common as a dependency
3. Use its httpgrpc to spin up a gRPC server
4. Translates the HTTP to gRPC, send to the HTTP server, and translates the response back

While I have not figured out how to implement step 4 in detail, just want to double-check this looks like a solid plan before digging in. Thanks!