add cabinet discovery and configuration

[PowellTravis]

• Extend inventory schema with a top‑level cabinets[] list.
• Implement DiscoverCabinets to extract cabinet xnames from BMC entries, query the Redfish manager (/Managers/BMC/EthernetInterfaces) for each cabinet, and populate MAC and IP (fallback to BMC values).
• Add ManagerInfo and DiscoverManagerInfo helpers in the redfish package to retrieve manager Ethernet interface details.
• Update discover CLI command to invoke cabinet discovery and apply SSH authorized‑key configuration to cabinets (re‑using the same key as for BMCs).
• Provide comprehensive unit tests:
  • cabinets_test.go validates Redfish manager discovery.

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Description

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Fixes #(issue)

Type of Change

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• New feature
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• Documentation update

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