[ECO][Inventory] Move APIs to entity_manager plugin

[cauemarcondes]

As part of our strategy to move away from materialized entities, we plan to expose APIs directly from the entity_manager plugin, rather than allowing consumers to query entities manually.

In the Inventory plugin, we currently query the entities-*-latest index directly, using ESQL, to power our UIs. We aim to start consuming the new APIs provided by the entity_manager, so that the UI remains agnostic to the underlying entity creation process.

These are the current inventory APIs we will start moving to the entity_manager plugin:

• Endpoint: GET /internal/inventory/entities/types
  Description: Fetch a list of available entity types.
  Return type:

z.array(
  z.object({
    type: z.string(),
    count: z.number(),
  })
)

• Endpoint: GET /internal/inventory/entities
  Description: Fetch all entities from a given entity type (group by)
  Return type:

z.array(entityLatestSchema)

• Remove the Unified entities page.

• We WILL NOT move the grouping API GET /internal/inventory/entities/group_by/{field}, we can use the entity types API for now since the UI only allows to group by entity type.

• We WILL NOT move the other APIs.

• We WILL NOT change how we're create data views to show the suggestion fields on the KQL bar.

• We WILL NOT move the logic to fetch alerts.

• The Inventory APIs will need to "convert" the entities received from the new services to the UI schema following what @crespocarlos created here.

[elasticmachine]

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

[cauemarcondes]

Closing this as the EEM team is moving fast and will have the services ready sooner than we expected, so no reason to move our stuff to their side.