Modify the Koji image extra metadata to not depend on the upload target
data structures for upload results. The target results are supposed to
be specific to the uploader implementation, which will eventually
change. Moreover, the definition is internal to osbuild-composer, so
this would create a problem once the Koji upload implementation is moved
to osbuild/images.
The Koji upload implementation does not really care about the exact
structure of the upload results. It is just a list of JSON objects.
Signed-off-by: Tomáš Hozza <thozza@redhat.com>