Draft: makepkg: Add option to automate PGP related tasks

Enabling it to automate safe PGP actions within makepkg. For now behing an opt-in feature flag because of feedback from irc.

Add autoimportpgp to automatically fetch the keys needed for validation from the default keyserver. Also export the minimal version of the signing key if it is not yet present. This ensures that we always have all the needed keys within the source package.