codeintel: Index queue API
Created by: efritz
This PR implements part of RFC 199: Arbitrary code execution in the auto-indexer and closes https://github.com/sourcegraph/sourcegraph/issues/12664.
This PR exposes additional endpoints in the precise-code-intel-indexer API that will allow downstream (internal) consumers to dequeue index job records for processing, mark them as complete or errored (possibly eventually streaming progress for visibility by end users), and keep actively processed jobs alive via heartbeat requests.