Resolve "Feature proposal: API for import from BitBucket Server"

What does this MR do?

The intention is to expose an API endpoint for the Bitbucket Server importer in order to allow programmatic and automated project imports.

The UI currently requires manual user input in order to list Bitbucket Server repositories, before cloning them to GitLab.

The API endpoint requires the same input in the form of a POST request payload.

Happy (201) and unhappy (422, 401, etc.) test cases are introduced.

A timed project import rate limit is applied. Details to be determined.

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

Closes #36788 (closed)