feat: add organization custom property resource

[oscarbc96]

Resolves #ISSUE_NUMBER

---

Before the change?

• The github_organization_custom_property resource did not exist.
• There was no way to manage GitHub organization-level custom properties via Terraform.

After the change?

• Added a new github_organization_custom_property resource.
• Supports defining name, type, allowed values, default value, requirement status, and edit permissions.
• Includes full validation logic using CustomizeDiff to ensure consistent and correct configurations.
• Supports custom marshaling to allow default_value to be a string, list, or null depending on the property type.
• A full test suite was added to validate all logic paths and prevent invalid configurations.

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes
• No

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[kfcampbell]

Have you run these tests? They do not pass for me. Example errors:

        Error: default_value must contain zero or one item when type is SINGLE_SELECT or STRING
        
          with github_organization_custom_property.test,
          on terraform_plugin_test.tf line 3, in resource "github_organization_custom_property" "test":
           3: resource "github_organization_custom_property" "test" {

        Error: default_value must be a subset of allowed_values
        
          with github_organization_custom_property.test,
          on terraform_plugin_test.tf line 3, in resource "github_organization_custom_property" "test":
           3: resource "github_organization_custom_property" "test" {

  | Error: default_value must be either "true" or "false" when type is TRUE_FALSE
  | 
  |   with github_organization_custom_property.test,
  |   on terraform_plugin_test.tf line 3, in resource "github_organization_custom_property" "test":
  |    3: resource "github_organization_custom_property" "test" {
  | 

[deiga]

@nickfloyd I think this might be a duplicate of: https://github.com/integrations/terraform-provider-github/blob/main/github/resource_github_organization_custom_properties.go

[nickfloyd]

@nickfloyd I think this might be a duplicate of: https://github.com/integrations/terraform-provider-github/blob/main/github/resource_github_organization_custom_properties.go

@deiga It looks like you're correct. It's unfortunate that the implemented one uses the plural naming scheme. The general pattern has been to use plural on collections and singular on single resource operations.

For instance, repository collaborators (single, collection) implements this approach.

I missed this one because we already had a plural named - single implementation 😬. Thats another thing that needs documentation - how to properly handle single operations verses plural ones.

I'm closing this and have added an issue to address this.

@oscarbc96 if you feel like this has been closed in error please @ me here an we can discuss. Thank you for the work you put in here.