Make Collector offline/inactive/expired thresholds configurable per fleet

These can be configured on a global level in the Collectors configuration at the moment. This global configuration should define the defaults but should be overridable per Fleet.

This came up a couple of times, like e.g. [here](https://github.com/Graylog2/graylog2-server/issues/26044#issuecomment-4542543662).

I've attached a spec for how this **could** work.

[2026-07-23-per-fleet-collector-thresholds-design.md](https://github.com/user-attachments/files/30310656/2026-07-23-per-fleet-collector-thresholds-design.md)

Start by reading the attached 2026-07-23-per-fleet-collector-thresholds-design.md spec and the discussion linked from issue 26044. Trace the existing global Collectors configuration and determine how the three thresholds should inherit defaults and be overridden per Fleet; done means each Fleet can use its own values while global settings remain the defaults.

Written by the indexing model from the issue text.