[Java][okhttp-gson] add echo client test (#14442)

* add echo api tests for java okhttp client

* test okhttp gson echo client in jdk8 workflow

* update samples

samples/client/echo_api/java/okhttp-gson/docs/PathApi.md

@@ -0,0 +1,73 @@
+# PathApi
+
+All URIs are relative to *http://localhost:3000*
+
+| Method | HTTP request | Description |
+|------------- | ------------- | -------------|
+| [**testsPathStringPathStringIntegerPathInteger**](PathApi.md#testsPathStringPathStringIntegerPathInteger) | **GET** /path/string/{path_string}/integer/{path_integer} | Test path parameter(s) |
+
+
+<a name="testsPathStringPathStringIntegerPathInteger"></a>
+# **testsPathStringPathStringIntegerPathInteger**
+> String testsPathStringPathStringIntegerPathInteger(pathString, pathInteger)
+
+Test path parameter(s)
+
+Test path parameter(s)
+
+### Example
+```java
+// Import classes:
+import org.openapitools.client.ApiClient;
+import org.openapitools.client.ApiException;
+import org.openapitools.client.Configuration;
+import org.openapitools.client.models.*;
+import org.openapitools.client.api.PathApi;
+
+public class Example {
+  public static void main(String[] args) {
+    ApiClient defaultClient = Configuration.getDefaultApiClient();
+    defaultClient.setBasePath("http://localhost:3000");
+
+    PathApi apiInstance = new PathApi(defaultClient);
+    String pathString = "pathString_example"; // String | 
+    Integer pathInteger = 56; // Integer | 
+    try {
+      String result = apiInstance.testsPathStringPathStringIntegerPathInteger(pathString, pathInteger);
+      System.out.println(result);
+    } catch (ApiException e) {
+      System.err.println("Exception when calling PathApi#testsPathStringPathStringIntegerPathInteger");
+      System.err.println("Status code: " + e.getCode());
+      System.err.println("Reason: " + e.getResponseBody());
+      System.err.println("Response headers: " + e.getResponseHeaders());
+      e.printStackTrace();
+    }
+  }
+}
+```
+
+### Parameters
+
+| Name | Type | Description  | Notes |
+|------------- | ------------- | ------------- | -------------|
+| **pathString** | **String**|  | |
+| **pathInteger** | **Integer**|  | |
+
+### Return type
+
+**String**
+
+### Authorization
+
+No authorization required
+
+### HTTP request headers
+
+ - **Content-Type**: Not defined
+ - **Accept**: text/plain
+
+### HTTP response details
+| Status code | Description | Response headers |
+|-------------|-------------|------------------|
+| **200** | Successful operation |  -  |
+