bsdinstall: add menu to install firmware

Add a menu to the installer to run fwget(8) inside the newly installed
system to install firmware known to be needed.
This requires working netowrking.

This is needed at least for wireless currently for when we entirely
stop shipping new firmware in src.git to have working networking on
the installed system (we already do need this for at least rtw89).

Sponsored by: The FreeBSD Foundation
Tested with: 4 different iwlwifi chipsets in a system (earlier version)
Suggested improvments by: jrtc27
Differential Revision: https://reviews.freebsd.org/D47491

(cherry picked from commit bbe2a1da2df639c616869aa838244c8094779bd4)