Chat Inbox (XK0)

[jazzz]

This PR adds a specification which describes a chat initialization protocol.

This replaces the previous effort #72 as the changes are substantial. This specification incorporates the feedback from the previous version - Specifically:

• Removes Waku specific details (such as ContentTopic) and introduces a delivery_address abstraction
• Removed inbox_address for simplicity
• Defers Conversation_id semantics to a implementation specific specification. #TODO: Link
• Defer envelope definition and framing strategy to implementation specific specification. #TODO: Link
• Adds clarity on MessageFlow

In addition it also:

• updates the encryption primitives to provide, mutual authentication.