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