Add custom user rules PU for Duo (chat, workflow, agentic chat)

What does this MR do and why?

This MR introduces a new PU for adding user-provided rules as an extra context item type (like we do for files, issues, dependencies, editor selection and so on)

References

Part of [Agentic Chat] Create custom rules - first iter... (gitlab-org/gitlab#550743 - closed)

How to set up and validate locally