[BIP Proposal] Add sp() output descriptor format for BIP352

In a recent message on the Bitcoin Development Mailing List, a suggestion was made regarding the design of wallet descriptors. The author, identified as Craig Raw, proposes an adjustment to the current structure of wallet descriptors. Specifically, he advocates for the introduction of an optional field within these descriptors to store users' birthdays. The motivation behind this proposal stems from the inconvenience and inefficiency associated with having to back up this information separately. Raw's argument is grounded in improving design practicality and user experience in managing wallet data. This recommendation reflects a broader discussion on enhancing the functionality and user-friendliness of digital wallets in the cryptocurrency domain.