Added a pre redirect notification hook

[dibyenduk]

Added a pre redirect notification hook which allows the application to handle the redirection. This helps when a form is posted to the application and the request is redirected to the Identity provider. In that case when the request returns from Identity provider, all the form post values are lost and the request is changed to a GET request. This hook will allow application to save the form post values in RedirectingtoIdentityProvider event and handle the redirection as post in the application.

[dnfgituser]

All CLA requirements met.

[Tratcher]

Please file an issue to discuss your scenario before we consider a PR. There may be other ways to handle this that don't require new API.

[dibyenduk]

@Tratcher - Created issue https://github.com/aspnet/AspNetKatana/issues/493