add external Key to custom type (#44418)

Co-authored-by: Hari K <kha@microsoft.com>

Author: hari9-9

specification/keyvault/data-plane/Keys/tspconfig.yaml

@@ -30,7 +30,7 @@ options:
     emitter-output-dir: "{output-dir}/{service-dir}/azure-security-keyvault-keys"
     namespace: "com.azure.security.keyvault.keys"
     models-subpackage: "implementation.models"
-    custom-types: "KeyAttestation,KeyCurveName,KeyExportEncryptionAlgorithm,KeyOperation,KeyRotationPolicyAction,KeyType,ReleaseKeyResult"
+    custom-types: "KeyAttestation,KeyCurveName,KeyExportEncryptionAlgorithm,KeyOperation,KeyRotationPolicyAction,KeyType,ReleaseKeyResult,ExternalKey"
     custom-types-subpackage: "models"
     customization-class: "customizations/src/main/java/KeysCustomizations.java"
     use-eclipse-language-server: false