Client ID doesn't support underscore #791
Description
Describe the bug
Currently client ID doesn't support underscore, which may block some of the existing tools of MQTT
To Reproduce
- Open https://mqttx.app/web-client
- Click "new connection", fill in host, port and path, click "connect"
You'll see the connection fails because the default client ID (mqttx_xxx) contains underscore.
If underscore is commonly used in existing MQTT tools, we should support it.