Allow listing custom source for RemoteRule (#122)
    
    Allows defining custom remote rule URLs in policies, for example:

    

        ---

        id: example_policy

        product_versions: [fedora-37]

        decision_context: bodhi_update_push_stable

        subject_type: koji_build

        rules:

          - !RemoteRule

            sources:

              - http://gating.example.com/gating1.yml

              - http://gating.example.com/gating2.yml