Validation for API property Model

[jonife]

Issue #, if available:

Description of changes:
Added validation to Api property Model to ensure that its either of type dict or an intrinsic function.

Description of how you validated changes:
if a customer provides in a SAM a value API property Model and its neither a type dict nor an intrinsic function. An error is raised notifying the customer of the invalid type.

Checklist:

• Add/update unit tests using:
  • Correct values
  • Bad/wrong values (None, empty, wrong type, length, etc.)
  • Intrinsic Functions
• Add/update integration tests
• make pr passes
• Update documentation
• Verify transformed template deploys and application functions as expected

Examples?

Please reach out in the comments, if you want to add an example. Examples will be
added to sam init through https://github.com/awslabs/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.