Inbound IAP Authentication Integration

[iamacarpet]

Validate the IAP headers as required to validate it is turned on.

Can we go further and integrate it into Laravel's user system?

[iamacarpet]

Two separate middlewares required, one for interactive users and one for service-to-service API style.

[iamacarpet]

Already using Google Auth with Laravel using socialite, which does integrate into the user system, so we'll just need to create a middleware to make it work IAP style if that's what we want, which shouldn't be too hard.

[iamacarpet]

This is done in the latest commit.

We've been able to allow Google to do most of the hard work for us and just validate against the Users API headers that they include, with a stateless check, integrated into Laravel's user system, that works for both web & API requests.