OSX Https certificate callbacks don't work

There was a change in .Net Core 2.0 for HttpClient on OSX which disabled using certificate validation callbacks. As WCF always sets a callback, all https requests will fail on OSX. We can add some workarounds to WCF to enable some scenarios. For OSX only, we need to have the following behaviors for each of the possible X509CertificateValidationMode values:

X509CertificateValidationMode | Behavior
-----------------------------------|----------
None | Use the special callback handler `HttpClientHandler.DangerousAcceptAnyServerCertificateValidator`
PeerTrust | Throw `PlatformNotSupportedException`. I think we already do for other reasons
ChainTrust | Set no callback as the default behavior is chain trust
PeerOrChainTrust | Throw `PlatformNotSupportedException`. I think we already do for other reasons
Custom | Throw `PlatformNotSupportedException`