Previously this type was falling back to using the deprecated apt-key(8) by
checking for existence of files/directories on the controller host in
gencode-remote.
Adding `--use-deprecated-apt-key` as an explicit boolean serves two purposes:
1. It prevents fallbacks that might end up doing the wrong thing
(as was the case)
2. It allows for a simple way to remove keys from the keyring that were
previously added with apt-key(8) to /etc/apt/trusted.gpg
This parameter is added marked as deprecated as is only intended use is to
migrate to directory-based keyrings as recommended by Debian for a few releases.
It will be removed when Debian 11 stops being supported.
During the review process of this merge request, it was noted that the state of
PGP Key Servers is somewhat suboptimal, that the examples encouraged bad
practise (it is trivial to produce collisions for short key IDs), and that
this use does not require the Web of Trust, but instead only the public key
that is signing the repository.
That is why this also adds `--source` as an argument allowing for in-type or
in-manifest provision of such public keys by the type/manifest maintainer and
the use of Key Servers is still supported, but discouraged.
