[feature]: SendPayment: allow specifying fee limits in terms of the asset being sent and not sats

[ZZiigguurraatt]

SendPayment (https://lightning.engineering/api-docs/api/taproot-assets/taproot-asset-channels/send-payment/) is for sending assets defined by an asset_id in a tapchannelrpc.SendPaymentRequest. Also must be defined in tapchannelrpc.SendPaymentRequest is payment_request, which is an object of type routerrpc.SendPaymentRequest from SendPaymentV2 (https://lightning.engineering/api-docs/api/lnd/router/send-payment-v2/). SendPayment is a wrapper for SendPaymentV2, so we need to specify fee_limit_sat in routerrpc.SendPaymentRequest in terms of sats.

However, from a user standpoint, they currently need to compute what the fee limit should be in sat, based a current quote from their channel peer that is exchanging their asset for sats. This can be confusing for the sender because they may not really have any sats to send as fees, and they don't want to send real sats, they only the asset they are trying to send to be used as fees. The sender also should not have to figure out what the current quote is going to be used for the current call to SendPayment and calculate an equivalent sats fee value in terms of the asset they are trying to send.

Please add a new field to tapchannelrpc.SendPaymentRequest, fee_limit_asset, and if present, override fee_limit_sat in payment_request.

Also, consider fixing lightningnetwork/lnd#7832 instead of, or in addition to this issue.