method to copy a PSBT input/output over to another PSBT

[niftynei]

Occassionally in c-lightning we need to re-combine PSBTs, e.g. when a funding transaction is being RBF'd and we need to add more inputs to an existing PSBT. Our wallet APIs return PSBTs, and we need a way to move inputs btw two psbt structs.

maybe something like

int psbt_copy_input(struct wally_psbt src_psbt, struct wally_psbt dst_psbt, size_t src_idx);

Where the src_idx is the input to copy over. They're added to the end of the dst_psbt.