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();

Empieza leyendo la API pública de String.java y la sección enlazada de RFC 7946 sobre Feature identifiers. Sigue dónde se define el tipo de id de feature y dónde está cubierto por tests; después, identifica los cambios de API necesarios tanto para strings como para números JSON. Se considera terminado cuando se admiten identificadores numéricos sin perder la compatibilidad existente con strings.