feat: [SQL] Do not register SQL if not supported in cluster

[kavilla]

Regardless if Observability or the SQL plugin installed in the cluster. We still register the language. [SOURCE]

Which lets the language be selectable but one selected bug like this gets reported:

Switch from DQL to SQL/PPL: can't search SQL/PPL anymore. To add onto this, I think something's wrong with the SQL/PPL endpoints. getting a Error: {"error":"no handler found for uri [/_plugins/_sql] and method [POST]"}

Expected results

SQL does NOT get registered and displayed in language selector

Action items

• Research approaches
  • some ideas but not limited to:
    • Using the capability service (example)
    • Alternative could be not starting the plugin unless Observability and SQL are installed with requiredEnginedPlugins [info]
    • use the dependences [example]
      • might not work because we are talking an engine plugin, might just have to HEAD request to _plugins/sql
  • in a world where data sources can be not limited to the local cluster. do we want to consider registering regardless and then within the data source work we have supported languages for data sources
• Implement
• Tests