Make target prim a kwarg for Reference/Payload code samples

[mati-nvidia]

It'd be easier to use these code samples if ref_target_path was optional. This is what it looks like now:

def add_ext_reference(prim: Usd.Prim, ref_asset_path: str, ref_target_path: Sdf.Path) -> None:
    references: Usd.References = prim.GetReferences()
    references.AddReference(
        assetPath=ref_asset_path,
        primPath=ref_target_path # OPTIONAL: Reference a specific target prim. Otherwise, uses the referenced layer's defaultPrim.
    )

A better interface might be add_ext_reference(prim: Usd.Prim, ref_asset_path: str, ref_target_path: Sdf.Path=None). This way if ref_target_path is None, you can use the alternate AddReference signature that doesn't use a target.

FYI @felixmey

[felixmey]

Yes agreed, also splitting of internal and external references in separate samples might help to get the references and payload samples streamlined