Sending channel, supports the following options:

• wechat - WeChat Official Account (default)
• app - App
• extension - Extension
• webhook - Third-party webhook (WeCom, DingTalk, Lark, ServerChan, IFTTT, etc.)
• clawbot - WeChat ClawBot
• cp - WeCom Application
• mail - Email
• sms - SMS
• voice - Voice

If not specified, WeChat Official Account channel will be used by default.