WIP: Add new git push_options.assignee

TODO:

• Confirm the security aspects
  • Someone that is not a developer in the project should not be able to assign an MR to anyone
  • It should not be possible to an assign an MR to a non-member in a private project
  • Are there other cases where a specific person cannot be assigned an MR?
• Multiple assignees should not work for CE

What does this MR do?

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

#28804 (closed)