Allow admin to overwrite repo URI schema (HTTP or HTTPS) in extrepo global config

Repo URIs are added to extrepo-data in the form that upstream gives them. However, the admin might want to overwrite this locally for example to allow those repos being cached by Apt Cachner NG for example. Another use case is to choose other mirror servers.

I am thinking of an interface like this in config.yaml:

---
repo_force_url_schema: http

The URI schemas that the upstream repos supports does not need to be tracked here because at least in my case, I configure in Apt Cacher NG to still hit the upstream repo using https. Just in my internal network, http is used.

See https://github.com/debops/debops/issues/683 for previous discussion.