feat(zypper): add repository discovery, add, and remove support

[Xarianne]

Add Zypper repository support

Summary

Implements full repository lifecycle support for the Zypper backend. Previously metapac only listed Zypper packages; it could not discover, add, or remove Zypper repositories. This change makes Zypper behave consistently with other backend repository implementations while respecting Zypper-specific conventions such as $releasever/$basearch variables and GPG key handling.

Changes

• src/backends/zypper.rs

  • Repository discovery now uses zypper repos --show-enabled-only --export -, which returns INI-style .repo definitions.
  • Discovered repositories are keyed by their Zypper alias.
  • Default openSUSE:* repositories are filtered out of unmanaged output.
  • ZypperRepoOptions gained an optional gpgkey field so exported signing-key configuration is preserved.
  • add_repos now supports two forms:
    • Direct .repo file URLs (e.g. Brave's official brave-browser.repo).
    • Regular baseurl repositories with an explicit gpgkey, which are recreated via a temporary .repo definition.
  • remove_repos removes repositories by alias using zypper removerepo.
  • Installed-package parsing now recognises the il (locked but explicitly installed) status in addition to i+, preventing locked packages such as kernel-default from appearing as missing.

• README.md

  • Replaced the Zypper "Standard usage." section with repository documentation and a PackMan/Brave example.
  • Removed a misleading $releasever paragraph.

• Tests

  • Added parser tests for exported repository definitions including gpgkey extraction.
  • Added a regression test for locked user-installed packages.

Example group file

zypper = {
  repos = [
    {
      name = "packman",
      options = {
        url = "https://ftp.fau.de/packman/suse/openSUSE_Tumbleweed/"
      }
    },
    {
      name = "brave-browser",
      options = {
        url = "https://brave-browser-rpm-release.s3.brave.com/brave-browser.repo"
      }
    }
  ],
  packages = [
    "brave-browser",
  ]
}

Notes for reviewers

• Discovery intentionally filters aliases starting with openSUSE: because these are distribution repositories managed by the openSUSE service. This is conceptually similar to DNF only listing COPR repos with dnf copr list; Zypper has no equivalent "third-party only" command, so a prefix filter is used to avoid cluttering unmanaged output with default repos.
• --non-interactive is only passed when no_confirm is set, matching existing backend conventions. Interactive metapac sync therefore lets Zypper prompt for new GPG keys.
• For discovered repositories with a gpgkey, metapac writes a temporary .repo file and passes that to zypper addrepo, because the addrepo command accepts either a plain URL plus alias or a full .repo file, not individual settings like gpgkey.