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