API tests further improvements.

[sobolk]

Follow up on https://github.com/aws-amplify/amplify-backend/issues/655 where initial version was developed.

TODO:

1. Further expansion to cover:
   1. inheritance
   2. exported consts
   3. See TODOs in https://github.com/aws-amplify/amplify-backend/blob/main/scripts/components/api-changes-validator/api_usage_statements_generators.ts
2. Integration with semver diffs/changeset. This check can be accompanied by semver diff check. I.e. a version bump coming from changeset can be validated against API diff detected (e.g. no API change - patch, non-breaking API change - minor, breaking API change - major, detection of no API change or intentional major version increment should skip this change for given package).
3. Mechanism to suppress the check for intentional breaks (for now check will be optional).
4. Consider publishing api-changes-validator as npm package.

[sobolk]

Class inheritance https://github.com/aws-amplify/amplify-backend/pull/876