feat: add organization custom property resource

[oscarbc96]

Resolves #ISSUE_NUMBER

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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" {
  |