main/postmarketos-config-nftables: add https rule, enable it upon nginx installation

firewall: Allow https traffic upon installation of nginx.

Would surely be useful to make this work with other webservers too, but didn't know how to write this into the apkbuild.

Main reason I'm contributing is this encouragement on the wiki:

A key goal of postmarketOS is to be polished. As such, if you install a package and it doesnt work without changing the nftables, you should contribute your nftables to pmaports. Contributing will make it easier to install the package on your other computers and help other postmarketOS users.

Closed because not everyone is using the same ports for their webserver. See Claytons comment for more detail.

Better place for change was Nextcloud installation guide on Alpine's wiki to remember people to configure their firewall.