[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/README.md

@@ -0,0 +1,147 @@
+# echo-api-okhttp-gson
+
+Echo Server API
+- API version: 0.1.0
+
+Echo Server API
+
+
+*Automatically generated by the [OpenAPI Generator](https://openapi-generator.tech)*
+
+
+## Requirements
+
+Building the API client library requires:
+1. Java 1.8+
+2. Maven (3.8.3+)/Gradle (7.2+)
+
+## Installation
+
+To install the API client library to your local Maven repository, simply execute:
+
+```shell
+mvn clean install
+```
+
+To deploy it to a remote Maven repository instead, configure the settings of the repository and execute:
+
+```shell
+mvn clean deploy
+```
+
+Refer to the [OSSRH Guide](http://central.sonatype.org/pages/ossrh-guide.html) for more information.
+
+### Maven users
+
+Add this dependency to your project's POM:
+
+```xml
+<dependency>
+  <groupId>org.openapitools</groupId>
+  <artifactId>echo-api-okhttp-gson</artifactId>
+  <version>0.1.0</version>
+  <scope>compile</scope>
+</dependency>
+```
+
+### Gradle users
+
+Add this dependency to your project's build file:
+
+```groovy
+  repositories {
+    mavenCentral()     // Needed if the 'echo-api-okhttp-gson' jar has been published to maven central.
+    mavenLocal()       // Needed if the 'echo-api-okhttp-gson' jar has been published to the local maven repo.
+  }
+
+  dependencies {
+     implementation "org.openapitools:echo-api-okhttp-gson:0.1.0"
+  }
+```
+
+### Others
+
+At first generate the JAR by executing:
+
+```shell
+mvn clean package
+```
+
+Then manually install the following JARs:
+
+* `target/echo-api-okhttp-gson-0.1.0.jar`
+* `target/lib/*.jar`
+
+## Getting Started
+
+Please follow the [installation](#installation) instruction and execute the following Java code:
+
+```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();
+    }
+  }
+}
+
+```
+
+## Documentation for API Endpoints
+
+All URIs are relative to *http://localhost:3000*
+
+Class | Method | HTTP request | Description
+------------ | ------------- | ------------- | -------------
+*BodyApi* | [**testEchoBodyPet**](docs/BodyApi.md#testEchoBodyPet) | **POST** /echo/body/Pet | Test body parameter(s)
+*PathApi* | [**testsPathStringPathStringIntegerPathInteger**](docs/PathApi.md#testsPathStringPathStringIntegerPathInteger) | **GET** /path/string/{path_string}/integer/{path_integer} | Test path parameter(s)
+*QueryApi* | [**testQueryIntegerBooleanString**](docs/QueryApi.md#testQueryIntegerBooleanString) | **GET** /query/integer/boolean/string | Test query parameter(s)
+*QueryApi* | [**testQueryStyleDeepObjectExplodeTrueObject**](docs/QueryApi.md#testQueryStyleDeepObjectExplodeTrueObject) | **GET** /query/style_deepObject/explode_true/object | Test query parameter(s)
+*QueryApi* | [**testQueryStyleDeepObjectExplodeTrueObjectAllOf**](docs/QueryApi.md#testQueryStyleDeepObjectExplodeTrueObjectAllOf) | **GET** /query/style_deepObject/explode_true/object/allOf | Test query parameter(s)
+*QueryApi* | [**testQueryStyleFormExplodeTrueArrayString**](docs/QueryApi.md#testQueryStyleFormExplodeTrueArrayString) | **GET** /query/style_form/explode_true/array_string | Test query parameter(s)
+*QueryApi* | [**testQueryStyleFormExplodeTrueObject**](docs/QueryApi.md#testQueryStyleFormExplodeTrueObject) | **GET** /query/style_form/explode_true/object | Test query parameter(s)
+
+
+## Documentation for Models
+
+ - [Bird](docs/Bird.md)
+ - [BirdAndCategory](docs/BirdAndCategory.md)
+ - [Category](docs/Category.md)
+ - [Pet](docs/Pet.md)
+ - [Tag](docs/Tag.md)
+ - [TestQueryStyleFormExplodeTrueArrayStringQueryObjectParameter](docs/TestQueryStyleFormExplodeTrueArrayStringQueryObjectParameter.md)
+
+
+## Documentation for Authorization
+
+All endpoints do not require authorization.
+Authentication schemes defined for the API:
+
+## Recommendation
+
+It's recommended to create an instance of `ApiClient` per thread in a multithreaded environment to avoid any potential issues.
+
+## Author
+
+team@openapitools.org
+