Add air gap support to OnPremises kind

[alessiodionisi]

Fixed some things in the schema that we found when working on clusters and added new schema variables.

Example:

spec:
  kubernetes:
    advancedAnsible:
      pythonInterpreter: python3
    advanced:
      airGap:
        containerdDownloadUrl: https://github.com/containerd/containerd/releases/download/v1.7.13/containerd-1.7.13-linux-amd64.tar.gz
        runcDownloadUrl: https://github.com/opencontainers/runc/releases/download/v1.1.12/runc.amd64
        runcChecksum: sha256:https://github.com/opencontainers/runc/releases/download/v1.1.12/runc.sha256sum
        etcdDownloadUrl: https://storage.googleapis.com/etcd
        dependenciesOverride:
          apt:
            name: k8s-1.27
            repo: deb https://pkgs.k8s.io/core:/stable:/v1.27/deb/ /
            gpg_key: https://pkgs.k8s.io/core:/stable:/v1.27/deb/Release.key
            gpg_key_id: DE15B14486CD377B9E876E1A234654DA9A296436
          yum:
            name: k8s-1.27
            repo: https://pkgs.k8s.io/core:/stable:/v1.27/rpm/
            gpg_key: https://pkgs.k8s.io/core:/stable:/v1.27/rpm/repodata/repomd.xml.key
            gpg_key_check: true
            repo_gpg_check: true

All new variables are optional with defaults.

Linked to https://github.com/sighupio/fury-kubernetes-on-premises/pull/78.

⚠️ Merge after https://github.com/sighupio/fury-kubernetes-on-premises/pull/78.