implement suggested exchange rate feature
currently, to make an offer, you must provide an exchange rate for the swap. this requires the user to manually calculate it themselves. it would be nice to add a subcommand for fetching a suggested exchange rate from an external API (coingecko or something like that?) this *would* be pretty centralized (unless we hit up many APIs and average them), so this should also notify the user to double check that they're okay with the rate.