Add IAP Support to Auth Options

[mawburn]

We use IAP (Identity Aware Proxy) for user authentication and have custom code for this that we need to put back in every time we upgrade.

It would be nice to just have this set as one of the supported auth strategies. We use the passport-http-header-strategy to accomplish this, so it doesn't need any extra packages to be added.

Me or someone from my team can take this feature on.