Public API Endpoint for updating APM Index Settings

[denizco]

Is your feature request related to a problem? Please describe.
Customer wants to update APM index settings (space-aware) programmatically via an API request. Currently, these settings can be changed in Kibana.

Describe the solution you'd like
Implement a public API endpoint to set APM index settings (in a space-aware manner)

Describe alternatives you've considered
There is an internal API endpoint, which can be reverse engineered, but we can't rely on this.

Additional context
https://elastic.slack.com/archives/C5G0S6QD7/p1728375392308979

[denizco]

Closing this one and opening up an issue in the Kibana repo: elastic/kibana#195399