Google IAP authentication

[pavgra]

Odysseus is going to add support for https://cloud.google.com/iap/ as an authentication mechanism for Atlas.

This auth method will differ from existing ones since there is no need to put credentials into the login form and validate them across external service. Google handles credentials verification on its own and passes information regarding the current user by putting additional headers to each request coming to WebAPI. So my suggestion for the feature design is following:

• Add setting determining whether it is Google IAP secured app OR regular app with classic auth
• If it is IAP secured app - replace current JwtAuthFilter with similar filter looking for Google's headers and extracting current user info from there