support updated RPA structures.

The type-4 payload was originally based on the BIP-stealth (https://github.com/genjix/bips/blob/master/bip-stealth.mediawiki) setup, and later considered the correct type for RPAs (https://github.com/imaginaryusername/Reusable_specs/blob/master/reusable_addresses.md), however the RPA paycode structure contains some extra fields not present in the original BIP-stealth and as such they are not compatible.

We should updated the specification to properly address the distinction between these two types and clarify the RPA structure.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information