Add Identity-Aware Proxy support

[bschaatsbergen]

Closes #34

what

• Added a new variable iap, that's used to configure Identity-Aware Proxy on a new backend.
• If the iap var is set, we route everything but /events over the IAP protected backend.
• /events is excluded as it's used to handle inbound webhook requests.

why

• Users should be offered an alternative to the basic auth authentication provided by Atlantis.
• Preferably a Google Cloud native authentication service.

references

• https://cloud.google.com/iap