code: public final B entityTag(BiFunction<String, HttpFileAttributes, String> entityTagFunction) {
          this.entityTagFunction = requireNonNull(entityTagFunction, "entityTagFunction");
          return self();
      }
docs: Sets the function which generates the entity tag that's used for setting the {@code "etag"} header
  automatically.
  
  @param entityTagFunction the entity tag function that generates the entity tag, or {@code null}
  to disable setting the {@code "etag"} header.