Add initial version of ci-flatdeb-builder

https://phabricator.apertis.org/T8509

Signed-off-by: Ryan Gonzalez ryan.gonzalez@collabora.com

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In draft status because this is still using my personal versions of the test repositories, pending the merges of tests/ci-flatdeb-builder-runtimes!1 (merged) and tests/ci-flatdeb-builder-apps!1 (merged). I would also ideally like some feedback on the configuration outlined in docs/apertis-website!476 (merged) first.