feat: protocol-aware envelopes

[jrriehl]

When a message sent is part of a protocol used by the agent, it can be useful to see this information in the envelope. Then the protocol manifest can be looked up from the digest, which gives much more context for a specific message.

In this PR:

• Envelope protocol field is now called schema_digest (keeping protocol as an alias)
• Adds optional protocol_digest field for the digest of the protocol to which the message belongs (if available)
• Context.send now tries to look up the protocol for a message before sending it
• Adds optional nonce field to allow for prevention of replay attacks
• This should be fully backwards compatible

[Alejandro-Morales]

Looks good 👍