postmarketos-config-nftables: add package for installing/configuring default fw

The reason for a new package with this name is so that the firewall 'implementation' can be changed and we don't have to create/destroy/replace packages to do that.

In this case, nftables is used, and some rules are included that:

1. drop all connections to wwan0 (more wwan names can be listed in this rule)...

2. allow ssh, drop from wwan (kinda redundant w/ the first rule, but doesn't hurt..)

fixes #1024 (closed)

depends on https://gitlab.com/postmarketOS/pmbootstrap/-/merge_requests/2042