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/.openapi-generator/FILES

@@ -0,0 +1,33 @@
+.github/workflows/maven.yml
+.gitignore
+.travis.yml
+README.md
+api/openapi.yaml
+build.gradle
+build.sbt
+git_push.sh
+gradle.properties
+gradle/wrapper/gradle-wrapper.jar
+gradle/wrapper/gradle-wrapper.properties
+gradlew
+gradlew.bat
+pom.xml
+settings.gradle
+src/main/AndroidManifest.xml
+src/main/java/org/openapitools/client/ApiClient.java
+src/main/java/org/openapitools/client/EncodingUtils.java
+src/main/java/org/openapitools/client/ServerConfiguration.java
+src/main/java/org/openapitools/client/ServerVariable.java
+src/main/java/org/openapitools/client/StringUtil.java
+src/main/java/org/openapitools/client/api/BodyApi.java
+src/main/java/org/openapitools/client/api/PathApi.java
+src/main/java/org/openapitools/client/api/QueryApi.java
+src/main/java/org/openapitools/client/auth/ApiKeyAuth.java
+src/main/java/org/openapitools/client/auth/DefaultApi20Impl.java
+src/main/java/org/openapitools/client/auth/HttpBasicAuth.java
+src/main/java/org/openapitools/client/auth/HttpBearerAuth.java
+src/main/java/org/openapitools/client/model/ApiResponse.java
+src/main/java/org/openapitools/client/model/Category.java
+src/main/java/org/openapitools/client/model/Pet.java
+src/main/java/org/openapitools/client/model/Tag.java
+src/main/java/org/openapitools/client/model/TestQueryStyleFormExplodeTrueArrayStringQueryObjectParameter.java

samples/client/echo_api/java/feign-gson/.openapi-generator/VERSION

@@ -0,0 +1 @@
+6.3.0-SNAPSHOT