Add new ChangeAnalysis /resourceChange and /changes API version 2021-…

…04-01 (#13285)

* Copy approved PR in private repo as baseline for new API version

* Add new API version 2021-04-01 for ChangeAnalysis

* Remove example files that no longer needed since we removd this API in stable version

* Remove produce default in swagger

* Resolve PR comments

* Add x-ms-secret tag on the oldValue and newValue property that could potentially contains secret value

* Remove the x-ms-secret tag, the swagger model validation reports error if we return this property, no matter if it's GET or POST request, while we very much want to return the values