Forward client certificates

[Tratcher]

If IIS accepts a TLS client certificate then HttpPlatformHandler should forward it on to us. This could be done as a base64 encoded header that is then unpacked by the middleware upon request from the ITlsConnectionFeature.

[davidfowl]

For Azure websites - https://azure.microsoft.com/en-us/documentation/articles/app-service-web-configure-tls-mutual-auth/#special-considerations-for-certificate-validation

[Tratcher]

Requires a new release of HttpPlatformHandler (v1.3?)