[Discover][APM] Refactor APM sources for better trace enablement (#212634)

## Summary

This PR reworks how APM handles getting its sources data, elevating the
necessary code to a private shared plugin so that Discover for Traces
can access the data and handle user provided configuration. It also
removes the need for Discover for Traces to rely on the APM static data
view, so the Trace data source and document profile will work on any
compatible/configured index, even in ESQL mode.

Closes #211414

<img alt="ESQL Discover Traces Screenshot 2025-03-04 173032"
src="https://github.com/user-attachments/assets/f5bbb736-8b8b-45dc-ac23-4bf7083aa47e"
/>

## How to test

Test with olbt-cli instance for now, will post for doing with synthtrace
data. Ensure the following is added to your kibana.dev.yml:

```yaml
discover.experimental.enabledProfiles:
  - observability-traces-data-source-profile
```

- Make sure your space has the Observability solution view configured
- Go to Discover page
- Select Data Views mode if required and create a view with a `traces`
specific index. Or use the APM static data view.
- The default columns on the page should show the summary column with
four of the following badges: `service.name`, `event.outcome`,
`transaction.name`, `transaction.duration.us`, `span.name`,
`span.duration.us`
- Go to ESQL mode with the query targetting a `traces` index
- The default columns should show the same as in Data View mode

---------

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>
Co-authored-by: Irene Blanco <irene.blanco@elastic.co>

packages/kbn-optimizer/limits.yml

@@ -5,6 +5,7 @@ pageLoadAssetSize:
   aiops: 32733
   alerting: 106936
   apm: 64385
+  apmSourcesAccess: 16765
   automaticImport: 25433
   banners: 17946
   canvas: 29355