Allow the assignment of alerts to users

What does this MR do?

This MR adds support for assigning alerts to users through the UI from the alert management detail view.

This includes the following items:

• Adds service to assign users to alerts
• Exposes assignee mutation via GraphQL API
• FE pieces - Approved & merged from !32955 (merged)

Out of scope: Multiple assignees. This is something we eventually will want to support, so this code keeps that in mind. But if multiple assignees are specified through the API, an error is returned.

Context

This MR represents a vertical slice of functionality for assigning alerts to users. This is slice 3/4.

This work will occur in 4 slices:

1. Create sidebar view for alert - !32642 (merged)
2. Add assignee to list view - !32609 (merged)
3. Add assignee dropdown to sidebar of alert detail view
4. Add system notes for alert assignment - MR TBD

Slices 2-4 will all be behind a alert_assignee feature flag. Grouping the work in this way will allow us to deliver functional chunks of work which can be validated individually.

Screenshots

Alert Management List	Alert details	Alert details selecting

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required) - !33069 (merged)
• 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

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