Add backend support for ai completion

What does this MR do and why?

This is the services layer extracted from !116583 (closed) so that it can be merged and unlock other teams

For Database review

There is nothing special on DB level in this MR. Some of the queries happening:

• Fetch user by id - code
• Fetch issue, epic, merge request by id - code
• Fetch first 100 notes for the above - code
• additional scope on notes to filter out internal and non-human notes - code
  • query plan: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/17708/commands/59138

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.