Debian extrepo support

From extrepo(1p):

The extrepo tool is used to manage external repositories in Debian. Before extrepo, users who wished to use software not packaged for Debian had to manually write the apt configuration files, run an unsigned script as root, or install an unsigned .deb package that contained all the configuration on their system. None of these methods were very secure.

Extrepo remedies this by way of a metadata repository for external package repositories. The user can search the list of metadata repositories by way of "extrepo search", and manage them through "extrepo enable", "extrepo disable", or "extrepo update".

Adding the repo to the official debian list (extrepo-data) should be easy, just need to create a pull request in their gitlab: https://salsa.debian.org/extrepo-team/extrepo-data/-/blob/master/README.md

Start with the extrepo(1p) description in the issue and the extrepo-data README linked in the body. Follow that repository's contribution instructions to add sing-box's external repository metadata; done means a pull request is created in extrepo-data.