Ability to create custom chains

[lukasic]

Hi. First of all, thank you for this awesome role!

Proposed feature allows creation of custom chains, in similar way how nft_input_rules / nft_input_group_rules / nft_input_host_rules works.

Example usage:

1. If you have IPv4 / IPv6 dualstack on docker server, you need to define all rules duplicitly in input and forward chains. With custom chain you can create for example docker chain and use jump docker in both input and forward.
2. If three levels (default / group / host) is not enough.

[kravietz]

@lukasic Thank you for your MR, it looks good but as we need to be sure it plays with the rest of the role in long term, could you also add test cases (see the molecule/ subdir for existing test cases).