.gitlab-ci.yml: publish RAUC bundle to HawkBit

The HawkBit instance needs to be configured externally and reachable from the runner performing the publishing step. The location of the instance must be provided through environment variables compatible with hawkbitctl, namely HAWKBIT_{URL,USERNAME,PASSWORD}. HAWKBIT_TENANT is optional and untested.

The bundle is added as an artifact to a new software module named after the bundle, using bundle timestamp as the version. The module is then placed in a distribution named after the image (not the bundle), with the exact same version.