Add idle Azure Kubernetes Fleet Manager check

[raphgm]

Reserved for the SKILL.SCH open source community.\n\nAKS Fleet Manager (used for multi-cluster management/update orchestration) has a Hub resource with its own fixed pricing tier once Standard/hub mode is enabled, separate from any member cluster costs. A Fleet hub with zero member clusters attached, or zero real update-run activity, is real waste on top of whatever the member AKS clusters themselves already cost.

[raphgm]

Resolved in commit db56fa3. Note: an earlier PR (#24) attempted this but merged a non-functional, fabricated version — SQL policy + tests only, no source/extraction file, no runner.py registration, and a flat $30/month price that doesn't reflect reality (AKS Fleet Manager itself is actually free per Microsoft's own pricing page). That version was replaced with a real implementation: the hub cluster's own AKS node infrastructure (Standard_DS3_v2 by default) is what actually bills, priced live via the Retail Prices API. Verified against a real Fleet hub with 0 member clusters attached -> $213.89/month flagged correctly.