[feature]: DecodeAssetPayReq: add option to determine asset amount based on RFQ and the edge node's buy rate

[ZZiigguurraatt]

DecodeAssetPayReq seems to decode the invoice, determines the sats amount, and then queries a nodes price oracle for their sell price to determine an asset amount that the invoice is representative of.

I'd like to see an option to also perform an RFQ with a peer to get the actual amount that would need to be sent to pay the invoice given the peer's buy price. This seems more representative of what the sender is going to actually need to send (less normal LN routing fees for sats channels).

I'd like to see the existing asset_amount that is returned kept but add this new information in another property such as asset_amount_RFQ. With this information, we can then compute the "implied fee" explained in #1367 in advance before attempting to make a payment.

[Roasbeef]

Note that there's no "true" RFQ amount. Let's say there's 20 peers we're connected to, and we do RFQ with each of them, which response do we show for decode pay req?

It's also important to note that as is, DecodePayReq is effectively a pure RPC, there's no state mutated, we just decode then query a price. Your suggested change would actually place a standing order, that may never actually be consumed. If a service is just doing decoding, these records (though they expire) start to build up.

[ZZiigguurraatt]

What are the drawbacks of having open RFQ records start to build? What is the drawback of a standing order that may never be consumed? I feel like #1358 and #1359 are going to produce the same kind of bloat.