[Bug] Grafana Plugin - variables not accepted in SQL "Drop Down List" query editor

### Search before asking

- [X] I searched in the [issues](https://github.com/apache/iotdb/issues) and found nothing similar.

### Version

IoTDB v1.3.2 in docker on MacOS

### Describe the bug and provide the minimal reproduce step

A variable for a device path has been created as per instructions.
This variable can be used successfully in the "SQL Full-Customized" query editor

As per the excellent documentation [1]

> Both SQL: Full Customized and *SQL: Drop-down List* input methods support the variable and template functions of grafana

However, trying to add the sensor path at the dropdown element shown as `"$endpoint"` below ...

[root] > [dropdown-sg] > "$endpoint"

... I get the following error:

> $endpoint is illegal, unquoted node name can only consist of digits, characters and underscore, or start or end with wildcard

Note: I have tried `$endpoint` `${endpoint}` and `{{endpoint}}` form of variable template syntax.

[1]: https://iotdb.apache.org/UserGuide/latest/Ecosystem-Integration/Grafana-Plugin.html#create-a-new-panel

### What did you expect to see?

Variable template syntax shown in path element instead of regular path component from dropdown

### What did you see instead?

Error

### Anything else?

I notice the source repo for the grafana plugin (iotdb-extras) does not have any issues tracked. I can re-submit there if required.

### Are you willing to submit a PR?

- [ ] I'm willing to submit a PR!

Beginne damit, das Problem im SQL Drop-down List-Editor des Grafana-Plugins mit IoTDB v1.3.2 zu reproduzieren und ihn mit dem SQL Full-Customized-Editor zu vergleichen. Untersuche anschließend den Plugin-Quellcode in iotdb-extras und die verlinkte Grafana-Plugin-Dokumentation. Als abgeschlossen gilt die Aufgabe, wenn eine Gerätepfadvariable im Dropdown-Editor akzeptiert wird, statt als ungültiger literaler Pfadknoten geparst zu werden.

Vom Indexierungsmodell aus dem Issue-Text verfasst.