Add TLS Handshake

[g-r-a-n-t]

The TLS handshake as described here https://github.com/libp2p/specs/blob/1a5e6eb805fc6c902128288ae1575945bcdc6e2a/tls/design%20considerations.md should be implemented.

[g-r-a-n-t]

Hey @tomaka, I sent you message on gitter inquiring about this work, but have not heard back, so I assume you don't check that.

Basically, I'm interested in contributing to libp2p and have heard that this functionality will be useful down the road. So if there aren't any major blockers on this, I'd like to take a crack at it.

[tomaka]

You can see some attempt from here in a branch: https://github.com/tomaka/libp2p-rs/blob/29e7b209c3cbdbdff2d7f2e737ea68c71eec4e0f/transports/quic/src/endpoint.rs#L161-L249

One of the main blocker is that the API of rustls doesn't have a straight-forward way of seeing the remote's certificate.
I temporarily used OpenSSL to generate the certificate, but I'd really appreciate if we didn't have to depend on OpenSSL just for this.

If you're motivated, go for it, but it's not a walk in the park.

[g-r-a-n-t]

I'll keep you posted.

[Demi-Marie]

I had to solve this as part of implementing QUIC.

[Geo25rey]

@tomaka It might be worth it to begin the switch to OpenSSL entirely since go-libp2p and go-ipfs have started this transition.