Event Grid: add support for 2021-10-15-preview API version (#18349)

* Using 2021-06-01-preview files as a baseline

* Add swagger, example, and readme files for 2021-10-15-preview API version

* Fix prettier errors and missing files

* Fix Python readme file

* Remove references to common-types files

* Fix missing brace in example file

* Fix ModelValidation errors

* Fix more ModelValidation errors

* Fix more ModelValidation errors

* Make systemData read only

* Run prettier for Channels_Update.json

* Fix various linter errors

* Data residency boundary fix

custom-words.txt

@@ -798,6 +798,7 @@ Geofence
 geofences
 Geofencing
 geojson
+Geopair
 georegions
 georeplication
 geospatial