The Sign method provides a way to sign a given Aleph message using an solana account. The full message is not used as the payload, only fields of the BaseMessage type are.
nacl is used to sign the payload with the account's private key. The final message's signature is composed of the signed payload and the user's public key.
The Aleph message to sign, using some of its fields.
SOLAccount implements the Account class for the Solana protocol. It is used to represent a solana account when publishing a message on the Aleph network.