feat(notif): clean up notification trait

[seanaye]

This PR cleans up the interface for notifications specifically:

• the safety around how we can construct a QueueMessage.
• The location of the rate limit configuration (non-optional for email notifications)
• making things more private if possible

• add port for digest state machine
• integrate state machine c with sns worker
• fix digest deserialization
• add batch poller to egress
• update queue message constructor
• update imports
• move rate limit into email
• update visibility
• fix tests