Given an unsigned PSBT, an array of signing public key(s) (one per input),
an array of signature(s) (one per input) in the same order as the pubkey(s),
adds partial signature object(s) to each input and returns the PSBT with
partial signature(s) included.
FIXME - maybe we add functionality of sending in a single pubkey as well,
which would assume all of the signature(s) are for that pubkey.
Given an unsigned PSBT, an array of signing public key(s) (one per input), an array of signature(s) (one per input) in the same order as the pubkey(s), adds partial signature object(s) to each input and returns the PSBT with partial signature(s) included.
FIXME - maybe we add functionality of sending in a single pubkey as well, which would assume all of the signature(s) are for that pubkey.