fix(sio): call adapter.init() when creating each namespace

The init() method of the adapter will now be called when creating a namespace with `io.of(<the-namespace>)`.

Note: any promise rejection is silently caught, as I don't see how we could properly expose the promise.

```js
const io = new Server({
  adapter: myAdapter
});
// under the hood, this:
// - implicitly creates the main namespace (/)
// - creates an instance of `myAdapter` for the main namespace
// - calls `myAdapter.init()` (with this change)
```

Related:

- #3662
- socketio/socket.io-postgres-adapter#16

Author: darrachequesne

packages/socket.io/lib/namespace.ts

@@ -197,6 +197,10 @@ export class Namespace<
   _initAdapter(): void {
     // @ts-ignore
     this.adapter = new (this.server.adapter()!)(this);
+
+    Promise.resolve(this.adapter.init()).catch((err) => {
+      debug("error while initializing adapter: %s", err);
+    });
   }
 
   /**