Represents an envelope for message communication between agents.
Attributes:
version (int): The envelope version.
sender (str): The sender's address.
target (str): The target's address.
session (UUID4): The session UUID that persists for back-and-forth
dialogues between agents.
schema_digest (str): The schema digest for the enclosed message.
protocol_digest (str | None): The digest of the protocol associated with the message
(optional).
payload (str | None): The encoded message payload of the envelope (optional).
expires (int | None): The expiration timestamp (optional).
nonce (int | None): The nonce value (optional).
signature (str | None): The envelope signature (optional).