Add MDE Onboarding API to Microsoft.Security (2021-10-01-preview) (Azure#16433)

* Rename 'whitelist'

* add missing 'Group' in renamed file

* add mdeOnboarding API - 2021-10-01-preview

* fix json format

* fix PR issues

* add List operation

* change to plural, set default resource type name

* mismatch between GET and LIST examples

* made descriptions clearer, removed unused parameter

* defined explicit parameters

* delete unused schema

Co-authored-by: Erel Hansav <[email protected]>

Author: erelh