Fix missing proxy override enum for managed instance swagger (#5780)

* Initializing instance pools swagger and examples - no usages / managed instances get by instance pool

* Fix missing enum for proxy override

specification/sql/resource-manager/Microsoft.Sql/preview/2015-05-01-preview/managedInstances.json

@@ -461,8 +461,17 @@
           ]
         },
         "proxyOverride": {
-          "description": "Proxy override of the managed instance.",
-          "type": "string"
+          "description": "Connection type used for connecting to the instance.",
+          "enum": [
+            "Proxy",
+            "Redirect",
+            "Default"
+          ],
+          "type": "string",
+          "x-ms-enum": {
+            "name": "ManagedInstanceProxyOverride",
+            "modelAsString": true
+          }
         },
         "timezoneId": {
           "description": "Id of the timezone. Allowed values are timezones supported by Windows.\r\nWindows keeps details on supported timezones, including the id, in registry under\r\nKEY_LOCAL_MACHINE\\SOFTWARE\\Microsoft\\Windows NT\\CurrentVersion\\Time Zones.\r\nYou can get those registry values via SQL Server by querying SELECT name AS timezone_id FROM sys.time_zone_info.\r\nList of Ids can also be obtained by executing [System.TimeZoneInfo]::GetSystemTimeZones() in PowerShell.\r\nAn example of valid timezone id is \"Pacific Standard Time\" or \"W. Europe Standard Time\".",