Add end-to-end encryption for uAgents messages

[jrriehl]

Research and implement basic end-to-end encryption for uAgents messages.

[Archento]

This feature will add communication overhead, i.e. latency to agent-to-agent communication so we'll implement it as an opt-out mechanism if an agent developer does not want or need to encrypt the traffic.

To be discussed:

• On what level do we apply encryption? (Message vs. interaction)
• What's the decisive factor going to be? (Common secret like DHKE, what key to use, etc.)
• How to enable configuration for the user
• How we implicitly handle the messages