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

In a recent discussion on the Bitcoin Development Mailing List, an intriguing approach to enhancing descriptor strings for third-party scanning servers was broached. The suggestion revolved around incorporating the birthday of the descriptor into the string itself. This addition would ostensibly provide scanning servers with the necessary information to determine the starting height for scanning without requiring external data inputs. Despite the perceived benefits, there are reservations regarding this proposal, particularly concerning the necessity and implications of introducing new key formats for this purpose.

The current consensus leans towards the adequacy of existing key formats in describing outputs and generating silent payment outputs through descriptors. An example provided illustrates how users can utilize their existing master key with a specific descriptor format to achieve silent payments. This approach negates the need for new key formats by ensuring that the descriptors remain unambiguous and straightforward, thus reducing potential confusion among users and wallet developers.

Further discussions touched upon the complexities inherent in descriptors with multiple keys and the comparative simplicity of the proposed "sp()" descriptor. The suggestion to eschew new key formats in favor of using a descriptor prefix for indicating silent payment versions was highlighted as a means to simplify the interface for both developers and users. This method would maintain the current workflow while integrating the new functionality seamlessly.

An additional point of interest was the proposal to encode multiple BIP352 labels within a single 64-bit number, thereby condensing the information into a more manageable form. This concept aims to address concerns regarding the variable length of descriptors that include arguments to scan for additional labels. By assigning bit positions corresponding to specific labels, the process becomes significantly streamlined, offering an efficient solution to the encoding of multiple labels without complicating the descriptor structure.

This ongoing dialogue underscores the community's commitment to evolving Bitcoin's technical infrastructure in a manner that balances innovation with user experience and system integrity.