Add work item email participants backend widget

What does this MR do and why?

Add /add_email and /remove_email quick actions to work items as per Work item quick actions: add_email and remove_e... (#462118 - closed)

Note: we don't need a post migration as agreed and documented !148119 (comment 1837432680) and https://docs.gitlab.com/ee/development/work_items_widgets.html#creating-a-new-backend-widget

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

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Before	After

How to set up and validate locally

1. Run migrations bundle exec rails db:migrate
2. Enable FF work_items_alpha
3. Create a new work item (issue, incident or ticket): http://gdk.local:3000/flightjs/Flight/-/work_items/new
4. Access the work item: http://gdk.local:3000/flightjs/Flight/-/work_items/:iid
5. Ensure the /remove_email quick action is NOT available
6. Ensure the /add_email quick action is available (and test it)
7. Ensure the /remove_email quick action is available (and test it)

Bonus points for ensuring the quick action isn't available:

• in the work item description when creating a new work item
• in other work item types
• in group level work items