feat: Add custom_properties to github_repository and github_enterprise_custom_property resource

Allow custom properties to be set on repositories at creation time, fixing 422 errors when an organization enforces required custom properties. Also adds a new github_enterprise_custom_property resource and data source for managing custom property definitions at the enterprise level.

Resolves #3230, #3304

Before the change?

• Organizations enforcing required custom properties would cause github_repository to return a 422 error on creation, since custom properties could only be set via a separate github_repository_custom_property resource applied after creation
• There was no Terraform resource or data source for defining custom property schemas at the enterprise level

After the change?

• github_repository now accepts a custom_properties argument (a string map) that is included in the initial repository creation request, satisfying organization-required custom properties at creation time
• A new github_enterprise_custom_property resource allows managing custom property definitions at the enterprise level, mirroring the existing github_organization_custom_properties resource
• A new github_enterprise_custom_property data source allows reading enterprise custom property definitions
• Documentation added for the new resource and data source, and updated for github_repository with a new argument description and example
• Acceptance tests added using testResourcePrefix naming and ConfigStateChecks pattern

Pull request checklist

• Schema migrations have been created if needed (example)
• 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?

• Yes
• No

Behavior Note About custom_properties on github_repository

This change adds a custom_properties attribute to github_repository that allows setting repository-level custom property values defined by the organization.

Example

resource "github_repository" "example" {
  name = "my-repo"

  custom_properties = {
    tier     = "Tier 4"
    team  = "platform"
  }
}

Behavior

Create — Custom properties are included directly in the POST /orgs/{org}/repos request body. This satisfies organizations that enforce required custom properties at repository creation time (which would otherwise reject the request with a 422).

Update — Changes are applied via the dedicated PATCH /repos/{owner}/{repo}/properties/values endpoint. The general repository update endpoint (PATCH /repos/{owner}/{repo}) silently ignores custom property changes, so the dedicated endpoint is required.

Read — Property values are refreshed from GET /repos/{owner}/{repo}/properties/values on each plan so out-of-band changes are detected as drift.

Config semantics

The last row is intentional and follows Terraform's Optional + Computed convention: org-required properties shouldn't be implicitly cleared, and accidental block removal shouldn't wipe values. Use custom_properties = {} to explicitly clear all values.