SdfCreateVariantInLayer is not consistent with UsdVariantSets .AddVariantSet

[jpankalainen]

Description of Issue

Currently the function SdfCreateVariantInLayer is not consistent with the default behavior of UsdVariantSets.AddVariantSet. The former currently adds the variant set name to the prim spec's list with the add verb, whereas the latter uses prepend by default and does not even have add as an option.

Steps to Reproduce

1. Create two prims to a stage with only one layer
2. Call SdfCreateVariantInLayer for a root layer prim spec for the first prim
3. Fetch the UsdVariantSets for the second prim and call AddVariantSet.
4. Export the layer as string and compare the results

System Information (OS, Hardware)

Windows 10

Package Versions

21.02

Build Flags

--no-usdview --no-imaging --build-monolithic

[jilliene]

Filed as internal issue #USD-6779