…elastic#199774)

## 📓 Summary

Related to elastic#198349

Disabling authorization on fields metadata `find API` implies every user
would have access to the integration fields since we use the internal
user to retrieve the package information.

On the other hand, requiring API root-level privileges for `fleet` and
`fleetv2` would restrict more use cases since other apps might rely on
this service to consume field metadata from ECS only, with no need for
integration permissions (Discover, etc.).

To keep the door open to all these use cases, we'll check the available
user privileges on a per-request basis and allow integration fields only
when they have access to fleet and integrations, without fully
restricting the service.

https://github.com/user-attachments/assets/49b9953a-f1e1-410a-8c7f-c38d87408fcc

---------

Co-authored-by: Marco Antonio Ghiani <marcoantonio.ghiani@elastic.co>
(cherry picked from commit 45056f3)