[tf.contrib.data] Switch backend Dataset representation to DT_VARIANT.

This change introduces a new `DatasetWrapper` type that wraps a
`DatasetBase*` and can be stored in a DT_VARIANT tensor. All Dataset
ops now consume and produce DT_VARIANT instead of DT_RESOURCE, and the
underlying implementation is simplified because the `DatasetWrapper`
can be passed directly by value without using the `ResourceMgr`.

PiperOrigin-RevId: 168240571