Alert assignee dropdown fe

What does this MR do?

A tiny MVC for: #218405 (closed)

In this merge we are adding the frontend work to allow a single assigee to be added to an alert. This supports the work already done on the list view screen to display in a column which assignee is an assigned to an alert.

Notes:

1. This merge is supported via this backend work: 63b47287
2. This merge requires work from !32642 (merged) which has yet to be merged but has been reviewed and approved(awaiting product approval).

The main FE files to review are:

app/assets/javascripts/alert_management/components/sidebar/sidebar_assignee.vue
app/assets/javascripts/alert_management/graphql/mutations/set_alert_assignees.graphql
app/assets/javascripts/alert_management/components/sidebar/sidebar_assignee.vue
app/assets/javascripts/alert_management/components/sidebar/sidebar_assignees.vue
spec/frontend/alert_management/components/alert_managment_sidebar_assignees_spec.js

Design Concept

Screenshots

List View	Details View	Collapsed View	Open View	Search View	No Results View	Mobile View

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
• 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