feat: answerfile kernel cmdline parameter

This could perhaps better belong in firstboot https://gitlab.alpinelinux.org/alpine/aports/-/blob/master/main/openrc/firstboot.initd, however from reading descriptions I know that keeping OpenRC close to upstream OpenRC is preferred.

In any event, this is a proposal to be able to provide a URI from which an answerfile is downloaded an automatically applied using setup-alpine -f answerfile. I am prepared to prepare an MR, if this is an acceptable idea.

Open questions:

1. Above (where should the code live)?
2. How do you wish the current requirement for manual intervention for root password (IIRC if an admin user is added in non-interactive mode, then there is no password on the account; this might wish a revisit as well). Likely requires revisiting answerfile in setup-alpine.
   1. Blank password
   2. No password but can SSH ('root:x:...` in /etc/shadow)
   3. Password locked
   4. Supply a password (or hash) to use.
   5. User picks which of the acceptable of these options to use
3. Would it be desirable to optionally provide a path instead of a URI (for example if a user provides the answerfile via a packages installed for the pkgs= cmdline parameter.

If it helps, I can work on a proof of concept with my answers to the above questions as draft MR, for further consideration.