Evolve EthDM to add reply with new chat key

[D4nte]

Problem

In the current version of Eth-DM (#204, vacp2p/rfc#403), Bob has no way to reply to Alice as Alice does not include any identifier in the Direct Message she sends.

Solution

Alice SHOULD include information in her Direct Message to allow Bob to reply with an encrypted message.
The following requirements apply:

• Only Bob should learn Alice's Ethereum Address
• Bob should be able to verify that Alice indeed owns her Ethereum Address
• Bob should only be able to find Alice's messages for which he is the recipient, ie, Alice should generate new Eth-DM keys for each recipient.
• Alice should be able to recover her Eth-DM keys from her Ethereum Account.

Notes

We may want to include a way for Bob to also share new, dedicated, Eth-DM keys to Alice in his first reply.

[D4nte]

Unnecessary at this stage, we are not aiming to re-doing Status chat app with ETh-DM.