fix: add missing nop logic for custom properties plugin

[PendaGTP]

Fixes #734

Implements proper nop (dry-run) logic handling for Custom PropertIes plugin, aligning with documentation and other plugins.

Changes

• Add nop mode support for property modifications
• Add paginate for gettings all custom properties
• Switch to built-in Octokit functions instead of github.request
• Update API endpoint format from /repos/:org/:repo/properties/values to /repos/{owner}/{repo}/properties/values for consistency with GitHub API
• Refactor property modification logic into a single modifyProperty method
• Add debug logging for better troubleshooting
• Normalize custom property names (from GH API) to lowercase to prevent case-sensitivity comparison issues
• Update test cases

Note

Currently, organization properties are not fetched from the GH Organization API. When modifying repo custom properties, this results in a 422 error if the property isn't defined at the org level. Future improvements could:

• Handle missing org properties to enable preview changes in nop mode
• Add support for defining Organization custom properties (requires broader changes)