[ReleasePR synapse] [DataFactory] Unify IR vNet injection experience (#1814)

Create to sync Azure/azure-rest-api-specs#14925
[ReCreate this PR](https://github.com/azure-resource-manager-schemas/compare/master...AzureSDKAutomation:sdkAuto/synapse?expand=1)

Author: msftbot[bot]

schemas/2020-12-01/Microsoft.Synapse.json

@@ -2831,6 +2831,10 @@
           "type": "string",
           "description": "The name of the subnet this integration runtime will join."
         },
+        "subnetId": {
+          "type": "string",
+          "description": "The ID of subnet, to which this Azure-SSIS integration runtime will be joined."
+        },
         "vNetId": {
           "type": "string",
           "description": "The ID of the VNet that this integration runtime will join."

schemas/2021-03-01/Microsoft.Synapse.json

@@ -2831,6 +2831,10 @@
           "type": "string",
           "description": "The name of the subnet this integration runtime will join."
         },
+        "subnetId": {
+          "type": "string",
+          "description": "The ID of subnet, to which this Azure-SSIS integration runtime will be joined."
+        },
         "vNetId": {
           "type": "string",
           "description": "The ID of the VNet that this integration runtime will join."

schemas/2021-05-01/Microsoft.Synapse.json

@@ -2852,6 +2852,10 @@
           "type": "string",
           "description": "The name of the subnet this integration runtime will join."
         },
+        "subnetId": {
+          "type": "string",
+          "description": "The ID of subnet, to which this Azure-SSIS integration runtime will be joined."
+        },
         "vNetId": {
           "type": "string",
           "description": "The ID of the VNet that this integration runtime will join."