feat: Add CEL rules to dependency proxy

[juliankepka]

Dependency proxy: CEL package rules

l3montree-dev/devguard#3104

Replaces the gitignore-style rules and the minReleaseAge setting with YAML-based CEL rules.

rules:
  - name: min-release-age
    action: deny
    expr: pkg.ecosystem != "oci" && now() - timestamp(pkg.releasedAt) < duration("120h")
  - name: allow-trusted
    action: allow
    expr: pkg.name == "my-trusted-package"

[juliankepka]

The Nix-tests fail because backend of main is not up to date with this branch. The merge of l3montree-dev/devguard#3180 is necessary