[JAVA][Feign] Replace Apache oltu with scribejava (#8318)

* - Replace apache oltu with scribejava
- Implement the following authentication methods
  - ApiKey header
  - HTTP basic authentication
  - Oauth client credentials flow
  - Oauth Implicit flow
  - Oauth Pasword (deprecated)

* Create class hierarchy for Oauth flows implementation

* Add instructions of how to use the ApiClient to Readme.md

* Update samples

* Remove support for java 6 and 7

* Remove java 6 and 7 support from gradle

* Format pom.xml

* Remove empty line

* Update samples

* Remove oltu dependency from build.gradle and build.sbt.
Replace oltu with ScribeJava

Update samples

* Update samples

* Update samples

Author: hugo-ma-alves

modules/openapi-generator-gradle-plugin/gradlew.bat

@@ -392,6 +392,10 @@ public void processOpts() {
             forceSerializationLibrary(SERIALIZATION_LIBRARY_JACKSON);
             supportingFiles.add(new SupportingFile("ParamExpander.mustache", invokerFolder, "ParamExpander.java"));
             supportingFiles.add(new SupportingFile("EncodingUtils.mustache", invokerFolder, "EncodingUtils.java"));
+            supportingFiles.add(new SupportingFile("auth/DefaultApi20Impl.mustache", authFolder, "DefaultApi20Impl.java"));
+            supportingFiles.add(new SupportingFile("auth/OauthPasswordGrant.mustache", authFolder, "OauthPasswordGrant.java"));
+            supportingFiles.add(new SupportingFile("auth/OauthClientCredentialsGrant.mustache", authFolder, "OauthClientCredentialsGrant.java"));
+
         } else if (OKHTTP_GSON.equals(getLibrary()) || StringUtils.isEmpty(getLibrary())) {
             // the "okhttp-gson" library template requires "ApiCallback.mustache" for async call
             supportingFiles.add(new SupportingFile("ApiCallback.mustache", invokerFolder, "ApiCallback.java"));

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/JavaClientCodegen.java

@@ -2,6 +2,10 @@
 
 package {{invokerPackage}}.auth;
 
+{{>generatedAnnotation}}
 public enum OAuthFlow {
-    accessCode, implicit, password, application
+    accessCode, //called authorizationCode  in OpenAPI 3.0
+    implicit, 
+    password,
+    application //called clientCredentials in OpenAPI 3.0
 }