Create YouTube "Radio" plugin

This should be a new plugin that is able to receive a YouTube URL from a user. The URL can either be to a single YouTube video or a video playlist. The plugin will receive the audio and play the audio on a voice channel.

The plugin will select which channel to play the audio by joining the voice channel the user is already connected to. This means the user must be connected to a voice channel prior to using the plugin.

首先阅读 repository 现有的 plugin 结构以及 Discord 语音处理的入口点。确定 plugin 应如何接受 YouTube URL、处理视频和 playlist、加入用户的语音频道并播放音频；对于已经连接到语音频道的用户，这些行为都能正常工作即表示完成。

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