[META] Change Inventory to consume new services exposed by EEM

[cauemarcondes]

TBD

• Feature flag to enable the new services API
• Keep the current queries in while testing the new services
• Use the new service to fetch a list of entity types. We'll use it in the grouped UI.
• Remove the unified page. There'll be no service to fetch all entities.
• Use the new service to fetch all entities from a given entity type (when a user opens an accordion on the grouped page). The entities inventory API (defined on the inventory plugin) should always return a common schema known by the UI which is defined here.
• Use the new service to get an adhoc dataview with all index patterns and use it in the KQL bar.
• We just moved to Control Groups here to filter by entity type, it automatically fetches the data that will be displayed on the field. It uses a dataview and a field name for this. I wonder how will that work now? cc: @simianhacker

[elasticmachine]

Pinging @elastic/obs-ux-infra_services-team (Team:obs-ux-infra_services)