feat: improve schema pass-through handling

[hoffa]

Issue #, if available

Description of changes

Follow-up to #3055; improving how we handle pass-throughs.

I realized we shouldn't typically use the CloudFormation documentation at the pass-through top-level, as the SAM documentation often includes useful SAM-specific information about the property (e.g. how it interacts with others).

So instead, we get the schema from CloudFormation, but use the documentation from SAM.

Description of how you validated changes

Checklist

• Adheres to the development guidelines
• Add/update transform tests
  • Using correct values
  • Using wrong values
• Add/update integration tests

Examples?

Please reach out in the comments if you want to add an example. Examples will be
added to sam init through aws/aws-sam-cli-app-templates.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.