public ShenYuTestCaseSpecBuilder addExists(final String endpoint, final String sendMessage, final String receiveMessage) {
            return add(WebSocketCheckers.exists(endpoint, sendMessage, receiveMessage));
        }
add exist method endpoint case spec.
@param endpoint endpoint
@param sendMessage sendMessage
@param receiveMessage receiveMessage
@return ShenYuTestCaseSpecBuilder
shenyu-e2e/shenyu-e2e-engine/src/main/java/org/apache/shenyu/e2e/engine/scenario/specification/ShenYuCaseSpec.java