Add support managed_challenge for custom_pages

[Nmishin]

For #1458

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[jacobbednarz]

we'll need to get the public documentation updated before we land this one - https://api.cloudflare.com/#custom-pages-for-a-zone-properties

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Marking this pull request as stale due to 14 days of inactivity. This helps our maintainers find and focus on the active pull requests. If this pull request receives no comments in the next 7 days it will automatically be closed. Maintainers can also remove the lifecycle/stale label.
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[clafollett]

We are in need of this PR being approved. Any way we can get this pushed through. We have managed challenges in production but can't back fill our TF configs to match and now our deployments error out.

[jacobbednarz]

i'm tracking the internal ticket for this and i'm hoping this will land in time for the next release.

[github-actions]

This functionality has been released in v3.13.0 of the Terraform Cloudflare Provider.

Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!