…erver side) (elastic#199638)

## Summary

This PR adds 2 new endpoints regarding enablement of the Entity Store:
* `api/entity_store/enable`, which initializes entity engines for both
`user` and `host` entities
* `api/entity_store/status`, which computes a global store status based
on the individual engine status

In addition, running initialization of multiple engines in parallel is
now allowed.

### How to test

1. Use dev tools to call `POST kbn:/api/entity_store/enable`
2. Check that two engines were created and that the status is
`installing` by calling `GET kbn:/api/entity_store/status`
3. Wait a few seconds and keep calling the `status` endpoint. Once
initialization finishes, the status should switch to `running`

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Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>
(cherry picked from commit 3757e64)

# Conflicts:
#	oas_docs/output/kibana.serverless.yaml
#	oas_docs/output/kibana.yaml