CodeGen from PR 16573 in Azure/azure-rest-api-specs

[Azure Stream Analytics] New 2021 preview version (#16573)

* copy 2017 folder

* add userAssignedIdenties

* update readme

* copy GA version to new preview folder

* update Identity

* update readme2

* some fixes

* missed one

* one more

* change 2020 to 2021

* hopefully no breaking changes

* address comments

* add functions

* add customClr

Co-authored-by: Roslyn Lu <roslu@microsoft.com>

Author: SDKAuto