This issue aims to track the implementation of the certificate and the certificate_request handshake messages for DTLS 1.3.

The new version of DTLS updates the structure of the Certificate (RFC 8446 section 4.4.2) and the CertificateRequest (RFC 8446 section 4.3.2) handshake messages. In DTLS 1.2 these are the defined in RFC 5246 section 7.4.2 and RFC 5246 section 7.4.4 respectively.

These new messages should be implemented as MessageCertificate13 in pkg/protocol/handshake/message_certificate_13.go and as MessageCertificateRequest13 in pkg/protocol/handshake/message_certificate_request_13.go.