Integrate gitlab-ci/report_image_changes_in_mr

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@1299a9d6

CI: add "report-image-diff" job

Add a python script and a GitLab CI job to run it. The job is triggered
when the build is complete.

The script retrieves build artifacts for the default/main branch
(currently "kirkstone") in the manifest repo and build artifacts
for the integration branch of merge request.

After that, it compares those builds and creates a summary of the overall
changes. In particular:
- size of compressed artifacts (artifacts.zip)
- size of built image
- difference between manifest files

The summary is reported to stdout and as a comment to the merge request.
Each run of the pipeline means removing the old summary comment and
creating a new one when the build is finished.