[RUN-AS-IF-FOSS] Abstract participants dropdown to a shared component

What does this MR do?

This MR implements two important separate changes:

1. We split app/assets/javascripts/sidebar/components/assignees/sidebar_assignees_widget.vue to separate the logic participants dropdown in the standalone shared component. This component will be used outside the sidebar (for example, in the Issue create view)
2. We change the logic to display users in the dropdown: instead of participants only, we also display project members (with inherited and invited groups)

How to test?

• Enable the assignees widget feature flag: Feature.enable(:issue_assignees_widget)
• Open any issue
• You should see the new component for issue assignees

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Related to #325944 (closed)