Add user mapping by username to Bitbucket Server importer

What does this MR do?

This MR adds optional usermapping by username to Bitbucket Server importer to allow some users to map users by username, when email is not available. This case can happen when migrating over from air gapped bitbucket server (behind firewall) to an intermediate proxy gitlab/bitbucket server where you lose some of the data on the way. Having optional usermapping by username should still allow users to migrate over their data with correct users assigned.

Mentions #218609 (closed)

Screenshots

Import by username

Feature.enable(:bitbucket_server_user_mapping_by_username)

Import by email

Feature.disable(:bitbucket_server_user_mapping_by_username)

Same behaviour as by username

Import when user does not exist

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

Closes #218609 (closed)