Support parsing: standalone vcpkg-configuration.json metadata

[mstykow]

Follow-up issue for first-class parsing/provenance support of standalone vcpkg-configuration.json files after the main vcpkg.json manifest parser lands.

Current status covered elsewhere:

• strict JSON parsing of vcpkg.json
• direct dependency extraction from the main manifest
• embedded configuration support via the manifest configuration field
• opportunistic ingestion of sibling vcpkg-configuration.json into manifest extra_data when no embedded configuration exists

Remaining work:

• decide whether standalone vcpkg-configuration.json should produce its own package-like record or a dedicated metadata model
• preserve registry/overlay provenance independently of vcpkg.json
• decide whether any assembly or sibling merge model is needed for separate configuration files

Related umbrella issue: #342