[FEAT]: Add github_actions_organization_permissions data source

### Describe the need

Currently, the provider supports the github_actions_organization_permissions resource, but there is no corresponding data source.

Organization-level Actions permissions are typically a global, "singleton" setting. When building distributed, modularized Terraform for repositories, it is often necessary to conditionally configure github_actions_repository_permissions based on the organization's overarching Actions policy (e.g., propagating the exact list of patterns_allowed).

Because there is no data source, Terraform authors must either hardcode these settings across environments or pass them down via complex variable chains from a root module.

Attempting to use the github_actions_organization_permissions resource inside a module with lifecycle { ignore_changes = all } as a workaround is dangerous, as destroying the module attempts to destroy the organization's permissions.

## Proposal
A new data source: `github_actions_organization_permissions`
This would wrap the existing `GET /orgs/{org}/actions/permissions` GitHub API and expose the exact same attributes as the existing resource:
- allowed_actions
- enabled_repositories
- allowed_actions_config (block)
- enabled_repositories_config (block)
- secret_scanning_push_protection_custom_link

Example usage:
```
data "github_actions_organization_permissions" "org_policy" {}

resource "github_actions_repository_permissions" "repo_policy" {
repository = "my-repo"
allowed_actions = data.github_actions_organization_permissions.org_policy.allowed_actions

allowed_actions_config {
github_owned_allowed = data.github_actions_organization_permissions.org_policy.allowed_actions_config[0].github_owned_allowed
verified_allowed = data.github_actions_organization_permissions.org_policy.allowed_actions_config[0].verified_allowed
patterns_allowed = data.github_actions_organization_permissions.org_policy.allowed_actions_config[0].patterns_allowed
}
}
```

The GetActionsPermissions method already exists on the OrganizationsService and is currently used by the resource's read function.

### GitHub Installation Type

- [ ] GitHub.com (Free, Pro, or Team)
- [x] GitHub Enterprise Server (on-premises)
- [ ] GitHub Enterprise Cloud with Personal Accounts (github.com)
- [x] GitHub Enterprise Cloud with Managed Users/EMU (github.com)
- [ ] GitHub Enterprise Cloud with Data Residency (*.ghe.com)
- [ ] I don't know

### Relevant log output

```shell
N/A
```

### Code of Conduct

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