Advanced Search: Add participant filter

Problem to solve

As a GitLab user, I should be able to filter issues by participants, so that I could quickly find all issues I'm involved in but not necessarily assigned to or authored.

Currently, there is no straightforward way to see all issues where a user is a participant. This makes it difficult to:

1. Track conversations you've contributed to
2. Follow up on issues you've commented on
3. Find previous discussions you've been involved in
4. Maintain context across multiple projects where you've participated

Users are currently resorting to "hacky" solutions or searching through email notifications to find issues they've participated in, which is inefficient and time-consuming.

Proposal

Add a new filter option to Advanced Search that allows filtering issues by participants.

The implementation would:

• Add the new field to the work_items index
• Add a new filter on the backend
• Add a new filter on the frontend
• Support users being able to quickly find all issues they've commented on, reacted to, or otherwise participated in

This feature is particularly valuable for:

• New team members trying to track their involvement across multiple projects
• Team members working across many different repositories
• Anyone who needs to follow up on previous discussions they've participated in

Technical considerations

• The participant filter should search across all participation types (comments, reactions, etc.)
• This feature could later be extended to Merge Requests as well