feat: Add support for Private Vulnerability Reporting

[eslerm]

Adds private_vulnerability_reporting to the github_repository resource's security_and_analysis block.

Implementation uses dedicated GitHub API endpoints rather than the repository Edit API, since PVR status is not included in the SecurityAndAnalysis response:

• EnablePrivateReporting/DisablePrivateReporting for writes
• IsPrivateReportingEnabled for reads
• Gracefully handles repos where PVR is unavailable

Changes:

• Add private_vulnerability_reporting schema field (Optional, Computed)
• Add updatePrivateVulnerabilityReporting helper function
• Fetch and merge PVR status separately in Read function
• Add acceptance test for public repository PVR management
• Add documentation for private_vulnerability_reporting field

Resolves #2399