[CDN] Fix customDomains property type in Endpoint (#19788)

* [CDN] Fix customDomains property type in Endpoint

* Fix linter error

Co-authored-by: Bo Zhang <bzhan@microsoft.com>

Author: t-bzhan

specification/cdn/resource-manager/Microsoft.Cdn/stable/2021-06-01/cdn.json

@@ -2657,8 +2657,11 @@
           "type": "array",
           "readOnly": true,
           "items": {
-            "$ref": "#/definitions/CustomDomain"
-          }
+            "$ref": "#/definitions/DeepCreatedCustomDomain"
+          },
+          "x-ms-identifiers": [
+            "name"
+          ]
         },
         "resourceState": {
           "description": "Resource status of the endpoint.",
@@ -5484,6 +5487,40 @@
         }
       }
     },
+    "DeepCreatedCustomDomain": {
+      "description": "Custom domains created on the CDN endpoint.",
+      "type": "object",
+      "required": [
+        "name"
+      ],
+      "properties": {
+        "name": {
+          "description": "Custom domain name.",
+          "type": "string"
+        },
+        "properties": {
+          "x-ms-client-flatten": true,
+          "$ref": "#/definitions/DeepCreatedCustomDomainProperties"
+        }
+      }
+    },
+    "DeepCreatedCustomDomainProperties": {
+      "description": "Properties of the custom domain created on the CDN endpoint.",
+      "type": "object",
+      "required": [
+        "hostName"
+      ],
+      "properties": {
+        "hostName": {
+          "description": "The host name of the custom domain. Must be a domain name.",
+          "type": "string"
+        },
+        "validationData": {
+          "description": "Special validation or data may be required when delivering CDN to some regions due to local compliance reasons. E.g. ICP license number of a custom domain is required to deliver content in China.",
+          "type": "string"
+        }
+      }
+    },
     "CustomDomain": {
       "description": "Friendly domain name mapping to the endpoint hostname that the customer provides for branding purposes, e.g. www.contoso.com.",
       "type": "object",