Mitigate problems with interdependence of GitHub importer and GitLab CI/CD for external repositories features

GitLab CI/CD for external repositories feature is owned by grouppipeline execution however it's functionality is partially based on and dependent on GitHub importer code.

As groupimport and integrate works on the GitHub importer, it might happen that changes to the importer affect also GitLab CI/CD for external repositories functionality. Additionally, it is harder to assign issues to the correct group. Current example:

• The import status remains in `started` indefini... (#419532 - closed)

This issue is to assess the problem and possible solutions that would enable:

• To have a clear ownership of the issues related to running GitLab CI/CD for external repositories functionality
• To avoid cases when we make change to importer code and unwillingly affect running GitLab CI/CD for external repositories functionality as well.

Possible solution could be to define and document the API that "running GitLab CI/CD for external repositories" expects, and cover it with tests.