2021 preview sb (Azure#13214)

* Add 2018 files as base commit

* Change files for 2021 in base files

* swagger fix lintdiff errors

* Add private endpoint connection property and refresh get example

* add python2 in readme

* Add checknamespaceavailability 2018 as base commit

* Change checknamespaceavaibility for 2021-preview

* fix track2 pipeline issue

* fix lintdiff error

* fix spell check

custom-words.txt

@@ -2112,3 +2112,34 @@ ziplist
 Zoho
 zoneinfo
 zset
+tpgs
+ProviderHub
+checkin
+regionality
+BRCPF
+IDRG
+EUGPS
+PLREGON
+ESDNI
+USUK
+Resource
+metdata
+Metastores
+maintenancewindows
+Logix
+Redirector
+destaging
+appendpos
+appendblock
+Exprired
+azureasyncoperations
+saskey
+vmsize
+CheckfeatureSupport
+Checkfeature
+lifecycles
+backuptype
+FSLogix
+privatelinkservice
+natgateway
+Ruleproperties