[FEAT]: Enterprise Runner Group Repository Selection

### Describe the need

We use the [github_enterprise_actions_runner_group](https://registry.terraform.io/providers/integrations/github/latest/docs/resources/enterprise_actions_runner_group) resource to create Enterprise Runner Groups & we want to configure those for use with selected repositories in specific Orgs. We use the `selected_organization_ids` field of that resource to select the required Orgs however we would like a way to automate the selection of repositories for Enterprise Runner Groups inside each Org.

The only relevant resource we found was [github_actions_runner_group](https://registry.terraform.io/providers/integrations/github/latest/docs/resources/actions_runner_group) which does have the ability to select repository IDs, however that one creates a new Org-level runner group of the same name.

Perhaps there should be a new resource that allows repository selection for Enterprise Runner Groups? e.g.

```
# prerequisite - an Org
data "github_organization" "example" {
name = "github"
}

# prerequisite - a Repo
resource "github_repository" "example" {
name = "my-repository"
}

# new resource type
resource "github_enterprise_actions_runner_group_repositories" "example" {
organization = data.github_organization.example.id
selected_repositories = [github_repository.example.repo_id]
}
```

That said, if I've missed some other way to add repository selections on an Enterprise Runner Group please do correct me.

### SDK Version

latest

### API Version

latest

### Relevant log output

```shell

```

### Code of Conduct

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