.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.

docs/ci/hidden-jobs/publish-rauc-bundle-to-hawkbit.rst

+.. SPDX-FileCopyrightText: Huawei Inc.
+..
+.. SPDX-License-Identifier: CC-BY-4.0
+
+===============================
+.publish-rauc-bundle-to-hawkbit
+===============================
+
+The `.publish-rauc-bundle-to-hawkbit` job uses `hawkbitctl` to publish a
+pre-build bundle to a HawkBit instance. The bundle is uploaded as an *artifact*
+to a new *software module*, which is then used to create a new *distribution
+set*.
+
+Usage Guide
+===========
+
+The `.publish-rauc-bundle-to-hawkbit` job should be paired with the
+:doc:`build-rauc-bundle-job` using the `needs: ...` and `dependencies: ...`
+keywords, so that the bundle file is transferred as an artifact between the two
+jobs, and no other artifacts are needlessly copied.
+
+The following variables should be set in the job definition:
+
+ - `CI_ONIRO_HAWKBIT_SWMOD_NAME` is the name of the software module to create.
+ - `CI_ONIRO_HAWKBIT_DS_NAME` is the name of the distribution set to create.
+ - `CI_ONIRO_RAUC_BUNDLE` is the name of the Yocto recipe responsible for the bundle.
+ - `MACHINE` is the Yocto machine variable.
+
+Note that the job relies on the presence of two artifact files: the bundle
+itself, which contains a timestamp-based version string, and the symbolic link
+pointing to it. The job that builds the bundle handles this automatically but
+custom jobs may need additional logic to preserve both files.
+
+The job will run automatically for tags, scheduled pipelines and for merge
+requests. Branch pipelines are excluded to avoid publishing extensive number of
+artifacts.