Integrate gitlab-ci/keep_artifacts

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@de9851be

ci: add a job to handle artifacts after merge

In the latest commit in the manifest it often happens, that the pipeline
does not contain the artifacts itself, but just a link to the pipeline of the MR.

Added a python script with corresponding GitLab CI job to launch this script.
The script searches for the latest pipelines for default branch in manifest repo,
and then decides either to keep or delete artifacts for particular build.

The build artifacts are kept for:
- the latest commit
- the last successful build
- the tagged commits (e.g. "fngsystem/47.0", "kirkstone/20.0")