message: allow specifying credentials for source registry on import image
diff: "type": "string"
  },
  "registryUri": {
  - "description": "The address of the source registry.",
  + "description": "The address of the source registry (e.g. 'mcr.microsoft.com').",
  "type": "string"
  },
  + "credentials": {
  + "$ref": "#/definitions/ImportSourceCredentials",
  + "description": "Credentials used when importing from a registry uri."
  + },
  "sourceImage": {
  "description": "Repository name of the source image.\r\nSpecify an image by repository ('hello-world'). This will use the 'latest' tag.\r\nSpecify an image by tag ('hello-world:latest').\r\nSpecify an image by sha256-based manifest digest ('hello-world@sha256:abc123').",
  "type": "string"
  }
  }
  },
  + "ImportSourceCredentials": {
  + "required": [
  + "password"
  + ],
  + "type": "object",
  + "properties": {
  + "username": {
  + "description": "The username to authenticate with the source registry.",
  + "type": "string"
  + },
  + "password": {
  + "description": "The password used to authenticate with the source registry.",
  + "type": "string"
  + }
  + }
  + },
  "RegistryNameCheckRequest": {
  "description": "A request to check whether a container registry name is available.",
  "required": [
  