[FEAT]: Support managing Dependabot malware alerts

### Describe the need

GitHub allows enabling **Dependabot malware alerts** on a per-repository basis (Settings > Advanced Security > Dependabot malware alerts). This setting controls whether Dependabot flags dependencies that contain known malware, separate from general vulnerability alerts.

Currently there is no Terraform resource or attribute to manage this setting. It can only be toggled through the GitHub UI.

A new resource (similar to `github_repository_dependabot_security_updates`) would allow teams to enforce this setting via IaC:

```hcl
resource "github_repository_dependabot_malware_alerts" "example" {
repository = github_repository.my_repo.name
enabled = true
}
```

### Why this matters

- Organizations managing security settings as Infrastructure as Code have no way to enforce or detect drift on this setting
- The GitHub REST API does not expose a per-repository endpoint to read or write malware alert status, so even workarounds using `null_resource` + API calls are not possible
- As supply chain attacks targeting dependency managers become more common, malware detection is increasingly important to manage at scale

### Workaround

None. The setting can only be managed through the GitHub UI. There is no REST API endpoint to read or write it at the repository level.

### SDK Version

N/A

### API Version

N/A

### Relevant log output

```shell
N/A
```

### Code of Conduct

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