Grant access to the public key of the secio layer

[tomaka]

Once we upgraded a connection through secio, the upper layer need to have somehow access to the public key that is used by the remote.

Required for a proper Kademlia implementation, otherwise we have no way to check the authenticity of nodes.

An easy solution would be to add a trait PublicKeyProvide with a get_public_key(&self) -> ? method, that would be implemented on the output of the secio middleware and on any stream that lies on top of secio. In my opinion it makes sense that this trait belongs to the core of the library (and not to libp2p-secio), in order to avoid a spaghetti of dependencies and also to enable alternative implementations of secio/security.

[tomaka]

Done a long time ago, I can't find it probably somewhere between PRs 150 and 250.