Add a new generic container registry client via a new `container`
package. Use this to create a command line utility as well as a
new upload target for container registries.
The code uses the github.com/containers/* project and packages to
interact with container registires that is also used by skopeo,
podman et al. One if the dependencies is `proglottis/gpgme` that
is using cgo to bind libgpgme, so we have to add the corresponding
devel package to the BuildRequires as well as installing it on CI.
Checks will follow later via an integration test.
RHEL requires the source code for dependencies to be included in the
srpm. The spec file already expects that, but we've only included the
vendored modules (i.e., the `vendor` directory) in the `rhel-8.2.`
branch. Move vendoring to master, so that we can build RHEL packages
from it as well.
This commit is the result of running `go mod vendor`, which includes the
vendored sources and updates go.mod and go.sum files.
Fedora requires the opposite: dependencies should not be vendored. The
spec file already ignores the `vendor` directory by default.
