[feature]: Extend the RFQ service to handle asset sell (send) initiated lightning payments

[ffranr]

The current RFQ service implementation supports asset purchase requests, enabling users to acquire tap assets at the end of a Lightning payment chain. This issue seeks to extend the RFQ service to encompass requests for asset sales. This enhancement enables users to settle a Lightning invoice by conducting an asset sale to the next hop in a Lightning payment route, based on mutual agreement.

Based on my assessment, the most straightforward method to accommodate asset sales within the RFQ service involves introducing new message types specifically designed for initiating and accepting asset sale requests. The existing request and accept message types will be designated for asset purchases only.

Upon completing this issue, the RFQ message types will be:

• SellRequest for initiating a sale,
• SellAccept for confirming the acceptance of a sale,
• BuyRequest for initiating a purchase,
• BuyAccept for confirming the acceptance of a purchase,
• Reject for declining any of the above requests.

Separate buy and sell request messages provides flexibility for the future, simplicity in naming and documenting fields, and unambiguous types in code.