feat: allow classes that inherit Resource to disable __setattr__ validation

[pradrx]

Issue #, if available

Description of changes

In the Resource class, we have an override on __setattr__ that checks if the attribute that is being modified is in the property_types dictionary or _keywords set (logical_id, relative_id, etc...).

Because of this, we cannot create class variables that can be modified after initialization. An example of why this can be annoying for developers is if you have a state that you would like to track throughout the class, you must do so internally with a local variable. This change adds a new boolean attribute to Resource, validate_setattr. If set to True (which is default value), then validation will be performed as normal. When it's false, it will bypass the property_types and _keywords check and allow you to modify other class variables without trouble.

Description of how you validated changes

Added unit tests where I created a dummy resource that inherits from Resource, with validate_setattr set to True and False in different cases.

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.