create steps & cleanup for nftables ruleset loading and cleanup

This would allow cleaner scenarios than what I use in !1101 (merged). Examples:

* Load nftables ruleset
  """
  table bridge filter {
      chain forward {
          type filter hook forward priority 0; policy accept;
              rule1 drop
              rule2 accept
          }
  }
  """

→ nft load ruleset

* Load nftables ruleset in "{ns}" namespace
  """
  ...
  """

→ ip netns exec nft load ruleset

Possibly also analogous cleanup steps (translating to nft flush ruleset). I'm not sure about need to clean up in temporary namespaces, clear info on how nftables interact with namespaces was hard to find for me...