bip321: add bip321 crate

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This PR adds a new crate to support BIP321, to parse and serialise payment URIs. It is ark-agnostic so it could be used outside of bark. In Bark, it will later enable parsing payment URIs and returning supported payment methods, along with estimate fees, greatly enhancing payment UX. It will also let application developers seamlessly generate payment URI to receive any kind of bark payments (onchain/lightning/ark)