[guardian] Add Guardian Address Signal Protocol (GASPv1) BIP draft

Implements the protocol for the Guardian Address proposal. The source code for the wallet implementation is available at https://github.com/bitcoinguardian/electrum. This serves as a model for self-custodial wallets, while custodial services may adapt the protocol to their infrastructure. Guardian Signal tooling is available at https://github.com/bitcoinguardian/GASPv1-draft A demo with testnet transactions and Guardian integration is available at https://github.com/bitcoinguardian/electrum/t

Specification