Support for SNS Subscription Filter Policies

I couldn't find a way to attach an SNS Subscription Filter Policy to a lambda function with chalice (there's no documentation on this in the [SNS Events section](https://aws.github.io/chalice/topics/events.html#sns-events) and the decorator doesn't seem to have an attribute for this).

Is there a way to do this with chalice?

Start with the SNS Events section in the Chalice documentation and the SNS event decorator, which the issue says lacks a subscription filter policy attribute. Determine whether Chalice can attach a filter policy to an SNS subscription for a Lambda function, then make the behavior available and document the resulting usage.

Written by the indexing model from the issue text.