PSET is missing `asset` field in pegin witness data

In PSET we have broken out all the fields of the `pegin_witness` field of a transaction input, *except* for the pegin asset type. I seem to recall the reason for this being that it was too complicated for Core to handle different pegin assets, and that no real blockchain would ever have multiple of these, so it had limited value.

However, for non-Elements finalizers, this field needs to exist, because it's used to construct the pegin witness.

It's fine if Elements rejects PSETs whose pegin assets don't match the consensus value, the field should just exist.

cc @achow101