Onboarding plugin as a required dependency on APM plugin in order to generate OTLP managed service URL out of APM managed service URL (by replacing .apm with .ingest).

As APM will be disabled for Logs Essentials, Onboarding needs to read OTLP managed service URL from observability plugin that's going to have managedOtlpServiceUrl in it's plugin config schema. This property will be populated for serverless clusters.

Changes needed:

• obervability plugin needs to expose managedOtlpServiceUrl property from its plugin contract
• Onboarding should remove the logic for generating OTLP service URL out of APM service URL and just read it from the Observability plugin
• APM dependency should be removed from Onboarding plugin

This issue has a couple of dependencies:

• [Observability] Add managedOtlpServiceUrl property to observability schema #225635
• https://github.com/elastic/project-controller/pull/1303