Add limited capacity worker for scheduled member deletion

What does this MR do and why?

In order to delete users from large hierarchies efficiently and without timeout, we are moving to an async process whereby we will add a limited capacity worker to process scheduled deletions.

A user will be scheduled for removal from a namespace hierarchy via the Members::DeletionSchedule table (added in !170268 (merged)).

In this MR, we add a limited capacity worker that will look up records in that table, then proceed to handle removal of matching Member records in the hierarchy in batches.

To err on the side of caution, we are also going to limit the worker to performing the removals in batches of 100 and limit execution time to 60 seconds.

We will keep the worker limited to 1 running job at any one time, which means we avoid the risk of saturating sidekiq with workers for 1000s of removals/memberships.

Refs #501248 (closed)

References

Please include cross links to any resources that are relevant to this MR This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• #501248 (closed) - issue for this change
• #501247 (closed) - issue for the related table
• #467281 (comment 2135360209) - issue/thread for discussion resulting in this approach

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Before	After

How to set up and validate locally

1. In rails console enable the experiment fully

   Feature.enable(:limited_capacity_member_destruction)

2. Visit any group or project member pages such as http://127.0.0.1:3000/groups/flightjs/-/group_members
3. Click the invite members button and add a user
4. Back in the rails console, create a scheduled deletion:

   member = Member.last # the membership you just created
   user = member.user
   source = member.source
   Members::DeletionSchedule.create(user: user, namespace: source.root_ancestor, scheduled_by: User.first) # replace User.first with your user if you wish

5. Next, we can schedule the new worker(s):

   Members::SchedulePruneDeletionsWorker.new.perform

6. Wait a few moments (for the workers to process) and confirm the user has been removed from the group/project hierarchy -->