The visibility of the "auth" property prevents it from being updated 

**auth** property is set as private and readonly.
Therefore, we can't update it as the documentation show us.

**To Reproduce**

Socket.IO client version: `4.0.0`

*Client*

I get a Typescript error here :
```ts
socket.auth.token = "efgh";
socket.disconnect().connect();
```

**Expected behavior**
We should have at least a setter to update this property.
