Add GET /grpc_status endpoint

[bilalebi]

I was working on this a while ago I didn't want it to go to wast

Description

This PR add an endpoint that can be used to check the health status of gRPC server

Knowledge Base

Official Docs of how it works: https://grpc.io/docs/guides/health-checking/

Example(s)

Request

http://0.0.0.0:8014/api/metadata/grpc_status

Response when it's UP (200)

{
  "status": 1,
  "code": "OK"
}

Response when it's DOWN (503)

{
  "status": 0,
  "code": "UNAVAILABLE"
}

Dependencies

This gRPC PR needs to be merged and deployed first: Ensembl/ensembl-metadata-api#127