code: public static String withKey(String key, Selector selector) {
          return String.format("\"%s\": %s", key, enclose(selector));
      }
docs: <P>
  Returns the string form of the specified key mapping to a JSON object enclosing the selector.
  </P>
  <pre>
  {@code
  Selector selector = eq("year", 2017);
  System.out.println(selector.toString());
  // Output: "year": {"$eq" : 2017}
  System.out.println(SelectorUtils.withKey("selector", selector));
  // Output: "selector": {"year": {"$eq": 2017}}
  }
  </pre>
  
  @param key      key to use for the selector (usually "selector" or "partial_filter_selector")
  @param selector the selector
  @return the string form of the selector enclosed in a JSON object, keyed by key