Support for Ad Hoc filters

**Is your feature request related to a problem? Please describe.**
Currently, when using the cloud logging data source plugin in grafana, we can not use Ad Hoc grafana filters to drive changes in the visualization.

[Grafana Ad Hoc vars](https://grafana.com/docs/grafana/latest/dashboards/variables/add-template-variables/#add-ad-hoc-filters)

**Describe the solution you'd like**
Ideally, I'd like to be able to click a log line in a log dashboard, click on the "+" or "-" button to create an Ad Hoc filter to show me matches (or non matches) of logs. Ex below:

This is supported by several other datasources. Loki [doc](https://grafana.com/docs/grafana/latest/datasources/loki/template-variables/#use-ad-hoc-filters) shows support for Ad Hoc filtering.

This would allow folks to drive queries natively in the dashboards without having to use explore or open/edit a dashboard to update/edit the query.

The issue names no files or tests; start by reviewing the plugin's Grafana data-source query and variable-handling entry points. Confirm how Ad Hoc filters are received and represented, then verify that clicking + or - on a log line changes matching and non-matching results in a dashboard.

Written by the indexing model from the issue text.