chore(recipe): Allow changing the public key used to verify base image

[EyeCantCU]

Adds an entry to recipe.yml enabling users to define their own public keys for the 'verify' action

[xynydev]

This feature desperately needs documentation somewhere. I'm not sure about a new top-level recipe entry, though... I mean, I can see the point, I'm just hesitant. But idk if this is still the better alternative to recommending the editing of build.yml.

Ok, how about making this an optional parameter, which uses the ublue key if nonexistant? So that people who ignore changes when doing manual merges to the recipe won't get a broken build. Then include this option in the recipe commented-out with a short explanation. This should also be added to the docs here: https://github.com/ublue-os/startingpoint/tree/template/config#basic-options