Function build_source 

pub async fn build_source(
    system: &SystemConfig,
    seed_path: &Path,
) -> Result<(Arc<dyn ConfigStore>, GatewayConfig, Option<PathBuf>), String>

Builds the etcd store and the initial gateway config.

Seeds etcd from the local gateway.yaml (seed_path) when the etcd prefix is empty, then loads from etcd. Returns config_path = None (etcd mode does not reload from disk).