Platform: Validate shared token for all queries

Problem to solve

Although the gRPC service is for internal-only (communicating within the Monolith infrastructure), we still need a level of authentication. Following a similar approach to Gitaly, we should create a shared ENV token which will be used by the gRPC service to authenticate the requests.

See how Gitaly does this.

Technical considerations

As per the design document, we must consider:

• We may want to use Bearer-style authentication headers for simplicity.
• We will be recording gRPC request metrics.
• Requests sent without a token should return 401 UNAUTHENTICATED.
• Token rotation will be handled in Infra: Add a runbook for how to rotate query AP... (#30).