feat. support setLocalVoiceChanger

[unclexiao]

目的：主要是针对 #23 提到的问题进行修复，新增了对语音变声效果的支持。

做法：根据SDK预设的6个选项，新增了枚举类型：

enum VoiceChanger {
  /// The original voice (no local voice change).
  VOICE_CHANGER_OFF,

  /// An old man's voice.
  VOICE_CHANGER_OLDMAN,

  /// A little boy's voice.
  VOICE_CHANGER_BABYBOY,

  ///A little girl's voice.
  VOICE_CHANGER_BABYGILR,

  /// Zhu Bajie's voice (Zhu Bajie is a character from Journey to the West who has a voice like a growling bear).
  VOICE_CHANGER_ZHUBAJIE,

  /// Ethereal vocal effects.
  VOICE_CHANGER_ETHEREAL,

  /// Hulk's voice.
  VOICE_CHANGER_HULK
}

并在invokeMethod方法转换为整形参数。

特别说明：需要升级io.agora.rtc:full-sdk依赖到最新版，才有支持这个方法。