Support swift-certificates types #568
Description
Higher-level packages sometimes need to use both swift-certificates and swift-nio-ssl, requiring conversions between types from both packages, e.g. X509.Certificate <-> NIOSSLCertificate, X509.Certificate.PrivateKey <-> NIOSSLPrivateKey. It would be good if swift-nio-ssl provided convenience helpers for these conversions. This would reduce boilerplate code and provide a canonical conversion path.