AlmaLinux vendors.d functionaloty rebased on top of v0.19

[Monstrofil]

This PR rebases vendors.d functionality (by @prilr ) and adds vendors.d/rpm-gpg folder as trusted gpg source root for successful upgrade with gpgcheck.

See also related leapp-data change:
AlmaLinux/leapp-data#62

[github-actions]

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[yuravk]

Looks good for now.