psbt: modify Extract method to return the transaction directly
psbt: modify Extract method to return the transaction directly

In this commit, we modify the Extract method to return the transaction
directly as in many cases a user will likely want to write the
transaction to disk, or perform additional validation rather than obtain
the raw bytes directly.

dajohi/btcutil,dajohi/btcutil,btcsuite/btcutil,btcsuite/btcutil