Send Talk message with NodeAddress only

[morph-dev]

In short, my understanding is that NodeAddress contains minimal information needed in order to send Talk message to another node.

I checked the code, but it doesn't seem so trivial. I'm willing to do required work and send PR, but I would appreciate some guideline on how to approach it.

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For context, I'm working on trin project and I'm trying to fix some issue. The issue itself is not related to discv5 itself, but the application protocol on top of it. The solution would be much easier if we could do what this issue describes (which I believe is doable in principle).

[ackintosh]

Unfortunately, I think we can't send a message using only NodeAddress, because the destination peer's public key is required when generating a session.

[AgeManning]

As @ackintosh points out, its not possible to send any messages without a node's public key.

The discv5 protocol requires us to encrypt the message with the node-id. We can't send anything without it.

Typically an ENR is required, but we made a function that works for multiaddrs, not that still has the public key:

discv5/src/discv5.rs

Line 520 in ab4ce9a

pub fn request_enr(