Add the information about osbuid artifact to the target result.
Specifically the name of the osbuild pipeline which was exported for the
specific target, and the filename of the exported file.
This will later enable embedding this information in Koji build metadata
to make it easy to reproduce the image build using the attached
manifest.
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.
Ensure that `UnmarshalTargetResultOptions()` is called only when there
are any options to unmarshal in the JSON object.
Since results of some Targets don't have any options defined, mark
`TargetResult.Options` as optional in the JSON tag.
Rather than having to assume that we only ever produce one
artifact, have each upload target contain the filename it expects
to upload from the osbuild output.
An image file is always explicitly named in the manifest, and we
leave it up to each distro to decide how this is done, but the
convention is to use the same image filename as used when
downloading the image through weldr.
Now make this policy explicit, by quering the distro for the image
name and inserting it into each upload target.
Signed-off-by: Tom Gundersen <teg@jklm.no>
