Support feature id as a number

Capturing from RFC

If a Feature has a commonly used identifier, that identifier SHOULD be included as a member of the Feature object with the name "id", and the value of this member is either a JSON string or number.

On a public API level we only expose String.java:

  /**
   * A feature may have a commonly used identifier which is either a unique String or number.
   *
   * @return a String containing this features unique identification or null if one wasn't given
   *   during creation.
   * @since 1.0.0
   */
  @Nullable
  public abstract String id();

先阅读公开的 String.java API，以及 RFC 7946 中关于 Feature identifiers 的链接章节。跟踪 feature id 类型的定义位置和测试覆盖情况，然后确定 JSON 字符串和数字两者所需的 API 更改。完成的标准是支持数字标识符，同时不丢失现有的字符串支持。