The osbuild export is specific to the upload target and different
targets may require using a different export. While osbuild-composer
still does not support multiple exports for osbuild jobs, this prepares
the ground for such support in the future.
The backward compatibility with older implementations of the composer
and workers is kept on the JSON (Un)mashaling level, where the JSON
message is always a super-set of the old and new way of providing the
exports to osbuild job.
The filename of the image as produced by osbuild for a given export is
currently set in each target options type in the `Filename` struct
member. However, the value is not really specific to any target type,
but to the specific export used for the target. For this reason move the
value form target type options to the `Target` struct inside a new
struct `OsbuildArtifact` under the name`ExportFilename`.
The backward compatibility with older implementations of the composer
and workers is kept on the JSON (Un)mashaling level, where the JSON
object is always a super-set of the old and new way of providing the
export filename in the Target.
