Add initial code handling certificate

[kpp]

Tracking issue: paritytech/polkadot-sdk#536

This is an initial implementation of a serializer, parser and verifier of x509 certificates with libp2p extension binary compatible with the go-libp2p-tls implementation.

cc @tomaka, @mxinden

[mxinden]

Thanks for the ping @kpp. In case you want to continue @tomaka 's work in branch https://github.com/tomaka/libp2p-rs/tree/quiccc-again you might want to cherry pick the commits from my fork https://github.com/mxinden/rust-libp2p/tree/quiccc-again "fixing" the unit tests.

[kpp]

Issues I found so far: x509-parser::X509Certificate::verify_signature:

1. is not aware of all signature algorithms we can actually support (like ed25519),
2. supports signatures with hash len < 256 like SHA1 (which is a big NO).

Shall I fix it in the current PR or shall I file issues and fix them in the future before actually integrating in the https://github.com/libp2p/rust-libp2p repo?

[kpp]

Will do in the next PR because we need to verify TLS certs & handshakes.