Dataset subject selection from API

Get the possible subjects for a dataset from the [Dataverse API](https://groups.google.com/g/dataverse-community/c/uPEyqd9BLc4/m/u1SAhd2UBgAJ?utm_medium=email&utm_source=footer), instead of using a hard-coded list (https://github.com/lepidus/dataversePlugin/issues/1).