Custom username for deploy tokens

What does this MR do?

Allow users to specify a custom username for deploy tokens that will be used instead of the default one (gitlab+deploy-token-#{id}). Use the specified username in Gitlab::Auth#deploy_token_check.

Empty usernames ("") will be saved as null in the database.

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

Performance 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

Related to #50228 (closed)