Migrate from DatasourceAPI to DatasourceWithBackend (#2123)

This PR migrates the use of `DatasourceApi` to `DatasourceWithBackend`,
with this a couple additional improvements were made:

1. Migrate to use `interpolateVariablesInQuery` everywhere instead of
the custom `replaceTemplateVariables` we were using
2. Moves util functions out of `datasource.ts` and into the existing
`utils.ts`

<img width="1261" height="406" alt="Screenshot 2025-11-20 at 11 37
56 AM"
src="https://github.com/user-attachments/assets/9e396cf2-eab0-49d1-958c-963a2e896eba"
/>

Now we can see the `query` calls being made to the backend:
<img width="367" height="102" alt="Screenshot 2025-11-20 at 11 38 18 AM"
src="https://github.com/user-attachments/assets/a5a9a337-7f19-4f7c-9d04-9d30c0216fb2"
/>

Tested:
- By running queries from Explore and Dashboards (with and without
variables)
- By interacting with all the different Editors to make sure `ComboBox`
was working as expected


Next:
Once this is merged, we will next be able to slowly move away from using
the `ZabbixConnector` to make backend datasource calls.

Fixes:
[#131](https://github.com/orgs/grafana/projects/457/views/40?pane=issue&itemId=139450234&issue=grafana%7Coss-big-tent-squad%7C131)

.changeset/tough-pugs-matter.md

@@ -0,0 +1,5 @@
+---
+'grafana-zabbix': major
+---
+
+Migrates use of DatasourceApi to DatasourceWithBackend