Support feature id as a number

[tobrun]

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();

[stale]

This issue has been automatically detected as stale because it has not had recent activity and will be archived. Thank you for your contributions.

[stale]

This issue has been automatically detected as stale because it has not had recent activity and will be archived. Thank you for your contributions.