[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/BodyApi.md

@@ -0,0 +1,71 @@
+# BodyApi
+
+All URIs are relative to *http://localhost:3000*
+
+| Method | HTTP request | Description |
+|------------- | ------------- | -------------|
+| [**testEchoBodyPet**](BodyApi.md#testEchoBodyPet) | **POST** /echo/body/Pet | Test body parameter(s) |
+
+
+<a name="testEchoBodyPet"></a>
+# **testEchoBodyPet**
+> Pet testEchoBodyPet(pet)
+
+Test body parameter(s)
+
+Test body 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.BodyApi;
+
+public class Example {
+  public static void main(String[] args) {
+    ApiClient defaultClient = Configuration.getDefaultApiClient();
+    defaultClient.setBasePath("http://localhost:3000");
+
+    BodyApi apiInstance = new BodyApi(defaultClient);
+    Pet pet = new Pet(); // Pet | Pet object that needs to be added to the store
+    try {
+      Pet result = apiInstance.testEchoBodyPet(pet);
+      System.out.println(result);
+    } catch (ApiException e) {
+      System.err.println("Exception when calling BodyApi#testEchoBodyPet");
+      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 |
+|------------- | ------------- | ------------- | -------------|
+| **pet** | [**Pet**](Pet.md)| Pet object that needs to be added to the store | [optional] |
+
+### Return type
+
+[**Pet**](Pet.md)
+
+### Authorization
+
+No authorization required
+
+### HTTP request headers
+
+ - **Content-Type**: application/json
+ - **Accept**: application/json
+
+### HTTP response details
+| Status code | Description | Response headers |
+|-------------|-------------|------------------|
+| **200** | Successful operation |  -  |
+