Add ability to test client cert configuration in the TestHttpClient

I would like to test some endpoints using client certifications.
It looks like api gateway supports it

https://aws.amazon.com/blogs/compute/introducing-mutual-tls-authentication-for-amazon-api-gateway/

maybe do it the way requests does it: https://stackoverflow.com/a/17604076
where you you can pass in a cert tuple to https://github.com/aws/chalice/blob/master/chalice/test.py#L109

and then the info gets added to the lamba event?

Start with chalice/test.py around line 109 and the TestHttpClient entry point, then compare the referenced requests certificate-tuple behavior with API Gateway mutual TLS documentation. Determine how client certificate configuration should be represented in the Lambda event; done means the test client can exercise an endpoint with that configuration and the resulting event is verifiable.

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