Allow changing the +salsaci in the automatic version bump in the CI

Hi,

This is more a feature request than a issue in fact.

Currently the only options options we have to regarding the automatic version bump in the CI is to enable (default behavior) or disable it. I think (well, I know because I'm advocating for its use :-) more people are using the Debian's pipeline to build packages in Gitlab CI outside Salsa and it would be cool to allow these "vendors" to customize their version name to be more like <upstream_version>-<debian_release>+${VENDOR_NAME}+${DATESTAMP}+${CI_PIPELINE_IID}.

I can submit a MR with a proposed change if you see it as a wanted feature. Something like:

-      sed -i -e "1 s/)/+salsaci+${DATESTAMP}+${CI_PIPELINE_IID})/" debian/changelog
+      sed -i -e "1 s/)/+${SALSA_CI_VERSION_BUMP_VENDOR_NAME}+${DATESTAMP}+${CI_PIPELINE_IID})/" debian/changelog

Cheers!