Reverse indexes based on agent's configuration

Problem to solve

For various GitLab Kubernetes Agent features a mechanism is needed that allows to lookup information based on fields/data in agent's configuration file. This issue is to devise such a mechanism.

Proposal

• Push configuration file updates from kas to GitLab when kas reads it for a connected agent.
• (Optional) Have a background job that queries kas for agent configuration e.g. once a day for inactive agents (those that hadn't authenticated for some time). This is just to keep the data up to date with configuration repo.
• Based on the incoming configuration, build required indexes. For example:
  • Manifest project id -> agent id index for #233631 (closed)
  • CI project id -> agent id and group id -> agent id indexes for &5528