Add API for suggesting unique alternatives if username exists

What does this MR do?

Adds a new API /users/WANTED_USERNAME/suggests Suggests unique alternatives if username exists.

This will be a drop in replacement for the /users/WANTED_USERNAME/exists

This API will be used by the following issue

• #32458 (closed)
• and the following #32458 (closed)
• TBD use in sign-ups, when a user creates a group or subgroup

Screenshots

Signup	group creation

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• 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