Validate signed message

A Message is a delta operation on the Farcaster network that represents a state change. Messages are the fundamental unit of data in Farcaster and can represent various actions like:

• Creating or removing casts (posts)
• Adding or removing reactions
• Following or unfollowing users
• Updating profile data
• Verifying Ethereum addresses

Each message contains:

• A MessageData object with the actual content
• A hash of the content for integrity verification
• A cryptographic signature to prove authenticity
• The signer's public key for verification

Messages are immutable once created and form an append-only log of all user actions on the network.