Consider adding `RFQ.payout.amount` field

[jiyoonie9]

In remittance use cases (perhaps trading too?), it is a pretty standard experience for Alice to get a Quote from the remittance provider in two different ways. For example, using USD > MXN currency pair, she might ask:

1. If I pay X USD, how much MXN will the recipient get?
2. If I want to send Y MXN, how much USD do I have to pay?

Currently, the RFQ allows Alice to only ask for the Quote per #1 above.

However, we should consider enabling #2 flow by adding RFQ.payout.amount field. The PFI should treat this as an XOR field so that Alice MUST send ONE of RFQ.payin.amount OR RFQ.payout.amount.

remitly_quote.mov