Add foreign outputs to `TxBuilder`

[LLFourn]

One use case for PSBTs is to be able to create a transaction where multiple parties have inputs. For example a dual funded lightning channel [1] where you are given the outpoint and scriptPubkey and value.
It would be nice if I could add this with TxBuilder and still let create_tx figure out what my inputs should be to make sure the joint output has the agreed upon total.