RFC 199: Expose internal routes for the indexer

Created by: efritz

The new indexer service will be deployed outside of the k8s cluster which means that it will not have access to the APIs that it needs out of the box.

We need to create a shared-token-authenticated proxy in the external frontend API that allows requests to a small set of gitserver endopints (for project cloning), as well as the index queue API defined in https://github.com/sourcegraph/sourcegraph/issues/12664.

See this slack thread for additional context.