[FEAT]: Support require_explicit_values for organization custom properties

### Describe the need

GitHub's organization custom-property API exposes `require_explicit_values`, but the `github_organization_custom_properties` resource and data source cannot read or set it. Terraform therefore cannot enforce that repositories choose an explicit property value or detect drift in that setting.

The underlying client also needs the field; this is tracked in google/go-github#4559. I plan to add the provider schema, request mapping, state refresh, tests, example, and generated documentation after a compatible client release is available.

GitHub API reference: https://docs.github.com/en/rest/orgs/custom-properties#create-or-update-a-custom-property-for-an-organization

### GitHub Installation Type

- [x] I don't know

### Relevant log output

```shell
N/A
```

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Start with the github_organization_custom_properties resource and data source, then check the compatible go-github release and google/go-github#4559 for client support. Once available, trace the schema, request mapping, state refresh, tests, example, and generated documentation; done means both resources can set and read require_explicit_values and detect drift.

Written by the indexing model from the issue text.