Enable an automatic external CI testing workflow for MRs:

1. push to MR ref, and ExternalCI::NeadsTesting labels are applied as appropriately by the kwf hooks, and the externalci comment is updated
2. CKI pipeline runs successfully
3. `eng.cki.ready_for_test` messages are sent, with external_ci=[...]
4. a KQE pipeline runs the tests (or somewhere far far away, a Jenkins job runs)
5. after that external pipeline finishes, an `eng.cki.results` message is sent back with the results
6. the ExternalCI labels in the MR are updated to either ::OK or ::Failed, and the externalci comment is updated

As a reference, the workflow should also work for a future where CKI testing is not special anymore:

• kernel CI pipelines only build kernels
• CKI test pipelines are triggered in KQE space via ready_for_test the same way as for other SSTs
• CKI test results are fed back into the MRs the same way as for other SSTs

—
Taken from https://gitlab.com/groups/cki-project/-/epics/110

• https://cki-project.org/docs/test-maintainers/testing-builds/
• https://groups.google.com/a/redhat.com/g/cki-project/c/r3eKstH2a6M

AC:

• [ ] sane extensible message format for ready-for-test and results, schema
• [ ] documentation
• [ ] communication with stakeholders

Jira: CKI-7054