BIP: Standard Encrypted Wallet Payload

[KeysSoze]

This pull request proposes a new BIP for a standard, interoperable format for encrypted wallet backups and transfers.

The core design choice is the separation of the data payload from the cryptographic container. This BIP specifies the payload format, while mandating that it must be encrypted by a container defined in companion proposals.

Key features include:

• CBOR Serialisation: Uses CBOR for a compact, efficient, and extensible binary format suitable for resource-constrained devices.
• Descriptor-Centric: Aligns with modern wallet architecture by using BIP 380 Output Script Descriptors as the primary method for defining accounts.
• Comprehensive Data: The schema includes accounts, master secret material, transaction data, UTXOs and standardised metadata.

This proposal was previously posted to the Bitcoin-Dev mailing list: https://groups.google.com/g/bitcoindev/c/3I-qtBhzIGY

Seeking assignment of a BIP number and welcoming all feedback.