Add dashboard variable to control AdHoc filter syntax

[SpencerTorres]

Fixes a regression from #1369 (reported in #1434) by allowing users to configure per-dashboard whether their OTel labels are Map or JSON type. Basically reverting the AdHoc filter changes and making them opt-in.

TL;DR: To "opt in" to JSON AdHoc filters, add clickhouse_adhoc_use_json as a constant variable (the value doesn't matter) to your dashboard.

Details

For OTel data, we merge and flatten ResourceAttributes, LogAttributes, and ScopeAttributes into a single labels field. The labels field then has values such as ResourceAttributes.host.name.

This means for JSON type, we can use the ResourceAttributes.host.name syntax directly, while for the Map type we must change the syntax to ResourceAttributes['host.name'].

Because the above Ad-Hoc filter doesn't specify type, we must add a way to configure this syntax to interpret as JSON or Map. We can't add it at the top level config, since it may be different per dashboard. So instead a user can simply add clickhouse_adhoc_use_json to enable the JSON syntax. If this is not present then it will default to Map.

Let me know if you have any thoughts on how we might improve this solution.

[aangelisc]

The changes LGTM but I'm uncertain on the enablement mechanism. I'd prefer updating the AdHocVariableForm in Grafana core to allow plugins to provide additional config 🤔