LUD21: A proposal for currencies on payRequest

BOLT 12 is highlighted for its capability to support non-native currencies, which includes an exchange rate negotiation mechanism between the sender and recipient. This feature allows the parties involved to agree on the number of millisatoshis (msats) that should be equivalent to a given amount of foreign currency. It's crucial for the integration and user experience that the semantics of LNURL and BOLT 12 are aligned. Any discrepancy between these two could lead to overlapping functionalities and result in confusion among users. For developers working with these protocols, it is imperative to ensure consistency and coherence to maintain a clear and straightforward user interaction process.