Make fetching of backports and proposed-updates repositories optional

The Apertis pipeline imports backport and/or proposed-updates versions by default, this is not needed and leads to miss some security updates. Because, importing versions from backports and proposed-updates is not part of our standard workflow, this should only be optional to avoid confusing the pipeline.

See infrastructure/ci-package-builder!316 (merged) for the ci-package-builder part.

https://phabricator.apertis.org/T9538

debian/changelog

+apertis-dev-tools (0.2023.11) apertis; urgency=medium
+
+  * apertis-pkg-pull-updates: Make fetching of backports and proposed-updates
+    repositories optional.
+    The Apertis pipeline imports backport and/or proposed-updates versions
+    by default, this is not needed and leads to miss some security updates.
+    Because, importing versions from backports and proposed-updates is not
+    part of our standard workflow, this should only be optional to avoid
+    confusing the pipeline.
+
+ -- Dylan Aïssi <daissi@debian.org>  Fri, 12 Jan 2024 14:48:38 +0100
+
 apertis-dev-tools (0.2023.10) apertis; urgency=medium
 
   * import-debian-package: avoid oversized commit messages