Provide more context to authorizer functions

The `AuthRequest` instance passed to the authorizer function doesn't provide any context about the request. This makes it impossible to implement rate limiting or blocking IPs that generate too many failed logins. Including the request context would make it possible to implement such protections.

Start by locating the AuthRequest definition and the authorizer invocation path in Chalice. Determine which request context is available there and define how it should be exposed; done means authorizer functions can use that context for protections such as rate limiting or blocking repeated failed logins, with tests covering the behavior.

Written by the indexing model from the issue text.