Add CloudFormationCustomResourceResponse struct.

[mawallace]

Description of changes:

This adds a CloudFormationCustomResourceResponse struct for responding to custom resource requests.

See the relevant documentation on the response format here: https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/crpg-ref-responses.html.

By submitting this pull request

• [x] I confirm that my contribution is made under the terms of the Apache 2.0 license.
• [x] I confirm that I've made a best effort attempt to update all relevant documentation.

[calavera]

Lint failures are unrelated to this PR. I'll fix them later.