Add an opt-in setting and the corresponding UI toggle to enable logs dataplane format.

Using this format allows Grafana correlations to work correctly with label values.
Without it, it's not possible to craft links that use label values when using Grafana correlations.

It's opt-in because the fields are renamed, which may break client-side transformations.

Summary by cubic

Adds an opt-in setting to output logs in Grafana dataplane format, enabling automatic ${labelKey} variables in correlations. Default is off to avoid breaking transformations that reference Time/Line.

• New Features
  • New “Use dataplane format” toggle in datasource settings; provisioning key: jsonData.useDataplaneFormat.
  • When enabled: rename Time→timestamp and Line→body, and set frame meta to DataFrameType.LogLines with typeVersion [0,0] so ${labelKey} works in correlations.
  • Updated README, CHANGELOG, and tests to cover the new format.

^{Written for commit e49b2ea. Summary will update on new commits.}