Add support for identity-less extension passing to facade classes

[http://blog.nerdbank.net/]

We need a way for RPs and OPs to pass redirect-based messages without including identity. The OpenID spec supports this, and the DNOA messaging stack supports it, but we don't expose it.

Proposed design:

OpenIdRelyingParty.CreateExtensionOnlyRequest()
Provider.IRequest -> interface derivative for handling only extensions

Accepted design:

var request = OpenIdRelyingParty.CreateRequest(string opIdentifier);
request.IsExtensionOnly = true;

[http://blog.nerdbank.net/]

master 9094ecb72a26fb509b8b774d3323f13b4dd6e60f