Add new Lambda infra for /auth

[john-osullivan]

The new Lambda fxn mounted at /auth is responsible for handling all auth interactions with Cognito, saving our users from needing to know anything but our API address. The state of that work is being maintained over here: https://github.com/Eximchain/terraform-aws-dappbot/pull/54

In a nutshell, I need:

• a new Lambda fxn
• a new IAM role
• a new policy granting sufficient Cognito access
• to mount new Lambda onto the API Gateway