Add the full gpg keys to the repository configs in the response.
On each repository object from dnf, the gpg keys are URLs, either
file:// or http(s)://. We need to resolve these and return them with
in the response.
When the URL is a file:// path, and it comes from a .repo config file,
we assume that the path is relative to the root_dir, so we prepend it to
the path in the file. This is so that repo configs in OS root trees can
be used unmodified. However, when a key is defined in the request, we
should assume that the path is valid, either because it was defined by
the caller as a URL, or because it was defined in-line in the request
and osbuild-depsolve-dnf wrote it to the persistdir itself.
A new exception is defined to identify errors during this process.
d2ce43ee50