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.
New upload target for VMWare, similar to the ones for AWS and Azure,
allowing users to set credentials for their vSphere instance.
Commit also includes function that performs the actual upload.
