Warning tooltip functionality in case of partial responses was accidentally removed

[ilyalabun]

Describe the bug

In version v0.7.0, commit 75899ee added frontend handling for
the isPartial response field. The old transform() function checked
response.data.isPartial and added a Grafana frame notice:

{
  severity: 'warning',
  text: 'The shown results are marked as PARTIAL...'
}

Later, commit 115d740 in v0.18.0 migrated query execution from the browser-side
implementation to the Go backend. After that change:

• Queries were sent through DataSourceWithBackend.
• The old frontend transform() function was no longer called.
• The Go backend became responsible for decoding VictoriaMetrics
  responses and creating Grafana data frames.
• The isPartial handling was not ported to the Go backend.

So we need to restore this functionality to give users explicit UI feedback when the data is not complete.

To Reproduce

• create mock data source which returns partial result
• try to create visualization using this data source
• you won't see any warning signs telling that data is not complete

Versions of VictoriaMetrics datasource and VictoriaMetrics backend

Plugin version: v0.18.0+

Link to dashboard in VictoriaMetrics grafana playground

No response

Please provide dashboard JSON if it is possible

No response

Additional information

No response

[github-actions]

🎉 The pull request #541 that addresses this issue has been merged!

PR Details:
- Title: fix: bring back warning about partial response
- Author: @ilyalabun
- Merged by: @arturminchukov
- Link: #541

This fix will be included in the next release.