Ability to sign PSBTs including timelocked outputs

[AdamISZ]

jmclient.wallet.PSBTWalletMixin only currently supports signing of standard scriptPubKey types - p2sh-p2wpkh and p2wpkh, i.e. our two Joinmarket wallet types. It doesn't support signing of any other scriptPubKey, and this would/will be needed for people intending to spend outputs created as p2wsh, as in the case of the now-implemented fidelity bond utxos, which use a timelocked custom script.