Salsa: implement three new commands: rename_branches, update_gbp_conf, file get/create/edit/delete/create_or_edit

This MR adds three independent commands. The goal is to add to salsa(1) the functionality required to perform mass-DEP14 migrations (or other operations) as discussed in https://lists.debian.org/debian-devel/2025/08/msg00560.html

The code in this MR was written with the help of AI (using the dep14-migrate python script as a basis). Please review with care. Also, my Perl is rusty, and I might not have gotten all the details about the implementation of salsa, so please review with even more care.

Note that a good review strategy could be to focus first on one of the tools.