Support for GSON Decoder in Java Feign Generator (#14254)

* Supporting Gson decoder in Feign

* Supporting Gson decoder in Feign

* Fixing test failures - and ensuring Jackson is used as the default if nothing selected (back compatible)

* Adding in sample files

* Updating docs

* Switching to echo server version

* Adding feign-gson to the github workflow

* Empty-Commit

samples/client/echo_api/java/feign-gson/README.md

@@ -0,0 +1,77 @@
+# echo-api-feign-json
+
+## Requirements
+
+Building the API client library requires [Maven](https://maven.apache.org/) to be installed.
+
+## Installation & Usage
+
+To install the API client library to your local Maven repository, simply execute:
+
+```shell
+mvn install
+```
+
+To deploy it to a remote Maven repository instead, configure the settings of the repository and execute:
+
+```shell
+mvn deploy
+```
+
+Refer to the [official documentation](https://maven.apache.org/plugins/maven-deploy-plugin/usage.html) for more information.
+
+After the client library is installed/deployed, you can use it in your Maven project by adding the following to your *pom.xml*:
+
+```xml
+<dependency>
+    <groupId>org.openapitools</groupId>
+    <artifactId>echo-api-feign-json</artifactId>
+    <version>0.1.0</version>
+    <scope>compile</scope>
+</dependency>
+
+```
+
+And to use the api you can follow the examples bellow:
+
+```java
+
+    //Set bearer token manually
+    ApiClient apiClient = new ApiClient("petstore_auth_client");
+    apiClient.setBasePath("https://localhost:8243/petstore/1/");
+    apiClient.setAccessToken("TOKEN", 10000);
+
+    //Use api key
+    ApiClient apiClient = new ApiClient("api_key", "API KEY");
+    apiClient.setBasePath("https://localhost:8243/petstore/1/");
+
+    //Use http basic authentication
+    ApiClient apiClient = new ApiClient("basicAuth");
+    apiClient.setBasePath("https://localhost:8243/petstore/1/");
+    apiClient.setCredentials("username", "password");
+
+    //Oauth password
+    ApiClient apiClient = new ApiClient("oauth_password");
+    apiClient.setBasePath("https://localhost:8243/petstore/1/");
+    apiClient.setOauthPassword("username", "password", "client_id", "client_secret");
+
+    //Oauth client credentials flow
+    ApiClient apiClient = new ApiClient("oauth_client_credentials");
+    apiClient.setBasePath("https://localhost:8243/petstore/1/");
+    apiClient.setClientCredentials("client_id", "client_secret");
+
+    PetApi petApi = apiClient.buildClient(PetApi.class);
+    Pet petById = petApi.getPetById(12345L);
+
+    System.out.println(petById);
+  }
+```
+
+## 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
+