Function execute_command 

pub async fn execute_command(
    backend: &Arc<dyn ExternalBackend>,
    gh: &GhHttp,
    repo: &str,
    task_id: &ExternalId,
    command: &OwnerCommand,
    store: &Option<Arc<dyn CommandStoreOps>>,
    task_manager: &Arc<TaskManager>,
) -> Result<String>

Execute a single owner command against a task.