Add support for dnf's sslcacert, sslclientcert, and sslclientkey
options. The latter two are passed as secrets (clientcert as well
because it might be a pem file that also includes the private key).
Sources run on the host, so their options may contain paths to the host
file system. Make use of that by accepting only paths in those options,
because it allows using tools to deal with certificate files.
Also make sure that the dnf source only returns options it knows about.
Pipelines encode which source content they need in the form of
repository metadata checksums (or rpm checksums). In addition, they
encode where they fetch that source content from in the form of URLs.
This is overly specific and doesn't have to be in the pipeline's hash:
the checksum is enough to specify an image.
In practice, this precluded using alternative ways of getting at source
packages, such as local mirrors, which could speed up development.
Introduce a new osbuild API: sources. With it, a stage can query for a
way to fetch source content based on checksums.
The first such source is `org.osbuild.dnf`, which returns repository
configuration for a metadata checksum. Note that the dnf stage continues
to verify that the content it received matches the checksum it expects.
Sources are implemented as programs, living in a `sources` directory.
They are run on the host (i.e., uncontained) right now. Each source gets
passed options, which are taken from a new command line argument to
osbuild, and an array of checksums for which to return content.
This API is only available to stages right now.
