Include members shared into project ancestors in issue assignee dropdown

What does this MR do and why?

Include members shared into project ancestors in the issue assignee dropdowns

The members shared into ancestors can be assigned using the /assign quick action. Here we include them in the issue assignee dropdowns on the issue list (<project-path>/-/issues) and issue (<project-path>/-/issues/<issue-number>) pages we well.

Screenshots or screen recordings

Here the user abc def is a member of foo/bar group.

Before	After

How to set up and validate locally

1. Create a group foo and within it create another group bar.
2. Add a member to foo/bar (abc def in the above example).
3. Create another group some-group and create a project within it project-a
4. Add foo/bar as a member of some-group.
5. Create an issue in project-a.
6. On some-group/project-a/-/issues, select Assignee is in the search bar and the member of foo/bar should appear after these changes.
7. On some-group/project-a/-/issues/<issue-id>, the Assignees dropdown should show the member of foo/bar after these changes.

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.

Closes #327825 (closed)