Add table github_repository_custom_property

[graza-io]

References
https://docs.github.com/en/rest/repos/custom-properties?apiVersion=2026-03-10

Requested in community Slack: https://turbot-community.slack.com/archives/C01UECB59A7/p1773759010669489

[github-actions]

This issue is stale because it has been open 60 days with no activity. Remove stale label or comment or this will be closed in 30 days.

[github-actions]

This issue was closed because it has been stalled for 90 days with no activity.