Adding a proof of funds

[gr0kchain]

Although the current BIP199 amendment and ERC1630 standards safeguard users from counter party risk through atomic swaps, it would be good if either party in the exchange was able to verify that they are dealing with a user who is in possession of the keys for the addresses involved in the swap. This mechanism helps ensure that there are no MITM attempts which aim to alter or hijack the swap from the confirming party of the swap.