[chore] Models are immutable

[nwithan8]

Description

• All setters for model properties are internal (end-user cannot modify any property of an object outside of an Update API call)

Testing

• No changes to unit tests
  • Test suite has internal access to source code, allowing it to use internal setters as needed during testing. End-users importing this library will not have internal access, and therefore won't be able to access the setters.

Pull Request Type

Please select the option(s) that are relevant to this PR.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement (fixing a typo, updating readme, renaming a variable name, etc)

[nwithan8]

Decided against this approach.