Expose `gitlab_employee?` to issue/MR assignee component

What does this MR do?

Closes #216650 (closed)

backend work needed to add the GitLab employee badge to the MR/issue assignee component

The following endpoints/serializers should be updated to include the is_gitlab_employee property on the user object.

• /issues/{issue_id}.json?serializer=sidebar_extras
• The value of this line
• /autocomplete/users.json
• /merge_requests/{mr_id}.json?serializer=sidebar_extras

is_gitlab_employee property is only relevant to gitlab.com, therefore the changes should be under ee folder !28375 (comment 324835821)

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
• 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