Per-route request-id generation/propagation override.

Modeled as a struct rather than Option<bool> on RouteMetadata because the wire shape is an object, not a bare boolean: fixtures/request_id.json's request_id_middleware_can_be_disabled sends {"enabled": false}, which Option<bool> cannot deserialize at all ("invalid type: map, expected a boolean") — the very fixture whose purpose is proving the middleware can be disabled was the one that failed to parse.