nftables: types: add conversion traits

nftables: types: add conversion traits

Some parts of the firewall config map directly to nftables objects, so
we introduce conversion traits for convenient conversion into the
respective nftables objects / types.

They are guarded behind a feature, so the nftables crate can be used
standalone without depending on the proxmox-ve-config crate.

Co-authored-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Signed-off-by: Stefan Hanreich <s.hanreich@proxmox.com>
Reviewed-by: Lukas Wagner <l.wagner@proxmox.com>
Reviewed-by: Max Carrara <m.carrara@proxmox.com>