[APM] Implement service map data endpoint for UI

[ogupte]

Related to: #44853

The UI for the service maps (#46497) was implemented and released behind config x-pack.apm.serviceMapEnabled: true but that UI uses a stubbed API endpoint with demo data. The endpoint /api/apm/service-map is implemented by executing an aggregation over documents which have connection fields populated by a background task.

POC: #42120

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)