feat: custom repositories.

[oxpa]

This PR adds (rather crude) support for third party repositories.

I'm trying to build a custom image for nginx (and later - nginx-plus).
Chisel seems like a good tool for the task. But it doesn't support third party repositories.

To work around this issue I added 'url' field into chisel.yaml. This allows one to add an archive like this:

  nginx:
    priority: 20
    version: 24.04
    components: [nginx]
    suites: [noble]
    public-keys: [nginx-key]
    url: https://packages.nginx.org/nginx/mainline/ubuntu/

And then add some slices into chisel-release to install from the new repository.
I'm not sure if I have to bump any versions. I would appreciate help with this.

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