[ReleasePR botservice] [BotService] Update Slack property (#2384)

Create to sync Azure/azure-rest-api-specs#19105
[ReCreate this PR](https://github.com/azure-resource-manager-schemas/compare/main...azure-sdk:sdkAuto/botservice?expand=1)

Author: msftbot[bot]

schemas/2021-05-01-preview/Microsoft.BotService.json

@@ -1838,6 +1838,17 @@
           "type": "string",
           "description": "The Slack landing page Url"
         },
+        "registerBeforeOAuthFlow": {
+          "oneOf": [
+            {
+              "type": "boolean"
+            },
+            {
+              "$ref": "https://schema.management.azure.com/schemas/common/definitions.json#/definitions/expression"
+            }
+          ],
+          "description": "Whether to register the settings before OAuth validation is performed. Recommended to True."
+        },
         "scopes": {
           "type": "string",
           "description": "The Slack permission scopes."