Multi signature pset signing support

[Saleciani]

Trying to sign a pset returned from sideswap api throws

ElementsMiniscriptPset(InputError(MissingPubkey, 0))

Sideswap creates a transaction from inputs we send them with several assets and inputs and outputs.

Trying to sign this transaction generates that error.

[Saleciani]

Update:

Problem with sideswap was unrelated to multi signature signing of transactions, but will keep issue open because multi signature is also important. :)