Add identity to messages

[atoulme]

It helps to add identity to the envelope, in the following cases:
Using UDP, as messages are dispatched.
Using TCP, if we don't rely on persistent connections.

In the case of RLPx, identities are exchanged during the handshake, and then again when exchanging hellos. That only works because we rely on TCP persistent connections.

[atoulme]

Proposal is to add a header to messages. The header is not required, but would be leveraged by application protocols such as the EWP RPC.

I would use the header under the key "signature".

I would also store the signature as a base64 encoded string suffixed by .sig and the curve name, similar to SSB.

                  IousK4cCn9T5qFa8E14GVek4cAMmMbjqDnAg==.sig.ed25519"```

```"z7W1ERg9UYZjNfE72ZwEuJF79khG+eOHWFp6iF+KLuSrw8Lqa6
                  IousK4cCn9T5qFa8E14GVek4cAMmMbjqDnAg==.sig.secp256k1"```