Payloads MUST be hashed using a cryptographic hash function

[nplasterer]

You should not be able to sign non hashed payloads

• payloads MUST be hashed using a cryptographic hash function as part of the ECDSA spec.
  • Some Crypto library API's, expected hashed strings so that implementors can choose which hash function to use in their cryptosystem
• Allowing developers to misuse hash functions in our library is dangerous, and not inline with defensible code practices.
  • XMTP sdks should only expose functions which expect raw strings
  • XMTPv2 uses two different hashing functions, so explicit functions should be created to ensure that developers never invoke sign on unhashed data

In particular this function https://github.com/xmtp/xmtp-ios/blob/main/Sources/XMTP/Messages/PrivateKey.swift#L33-L41 needs to explicitly disallow being called without a hashed payload.