Microsoft.ApiManagement Services_get operation - etag is missing in response model

The service is returning an etag, but the [swagger](https://github.com/Azure/azure-rest-api-specs/blob/0bef2b99ad6d52781e3c308bc6b420f6eef59ef7/specification/apimanagement/resource-manager/Microsoft.ApiManagement/stable/2016-10-10/apimdeployment.json#L1022) is missing said property. For most languages, this means that the property will be silently dropped by our generated SDKs. In addition, it will not be documented in our REST API documentation.