Add new ChangeAnalysis /resourceChange and /changes API version 2021-04-01 (Azure#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

Author: yantang-msft