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

The advancement in silent payments within the Bitcoin ecosystem is aimed at enhancing wallet interoperability and ensuring efficient backup and recovery processes. This initiative follows prior discussions, building upon them with a new proposal presented through a Bitcoin Improvement Proposal (BIP) found at https://github.com/bitcoin/bips/pull/2047. The proposal introduces a novel top-level script expression, sp(), which serves as a foundation for this mechanism.

Central to this proposal are two key expressions: spscan1q and spspend1q. The first encapsulates the scan private key alongside the spend public key, while the latter combines the scan private key with the spend private key. These expressions facilitate the generation of outputs by amalgamating this specified key material with sender input public keys, aiming to streamline transaction processes.

To alleviate the scanning workload, the proposal allows for an optional specification of a block height within the sp() expression. This inclusion acts as a marker for a wallet's "birthday," potentially optimizing the scanning process. Additionally, the framework permits the specification of zero or more positive integers as arguments within the sp() expression, intended for scanning additional BIP352 labels. Notably, the change label (m = 0) is automatically included, underscoring the proposal’s emphasis on efficiency and user convenience.

Provided examples within the message illustrate the practical application of these concepts, showcasing how these new expressions can be utilized in various scenarios to facilitate silent payments. This development signifies a pivotal step towards achieving greater interoperability among wallets, along with enhancing the security and reliability of backup and recovery operations within the Bitcoin network.