[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

samples/client/petstore/java/feign-no-nullable/pom.xml

@@ -154,7 +154,7 @@
                 <version>3.1.1</version>
                 <configuration>
                     <doclint>none</doclint>
-                        <source>1.7</source>
+                    <source>1.8</source>
                 </configuration>
                 <executions>
                     <execution>
@@ -257,15 +257,15 @@
             <artifactId>jackson-databind</artifactId>
             <version>${jackson-databind-version}</version>
         </dependency>
-            <dependency>
-                <groupId>com.github.joschi.jackson</groupId>
-                <artifactId>jackson-datatype-threetenbp</artifactId>
-                <version>${jackson-threetenbp-version}</version>
-            </dependency>
         <dependency>
-            <groupId>org.apache.oltu.oauth2</groupId>
-            <artifactId>org.apache.oltu.oauth2.client</artifactId>
-            <version>${oltu-version}</version>
+          <groupId>com.github.joschi.jackson</groupId>
+          <artifactId>jackson-datatype-threetenbp</artifactId>
+          <version>${jackson-threetenbp-version}</version>
+        </dependency>
+        <dependency>
+            <groupId>com.github.scribejava</groupId>
+            <artifactId>scribejava-core</artifactId>
+            <version>${scribejava-version}</version>
         </dependency>
         <dependency>
             <groupId>javax.annotation</groupId>
@@ -296,7 +296,7 @@
     </dependencies>
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
-        <java.version>1.7</java.version>
+        <java.version>1.8</java.version>
         <maven.compiler.source>${java.version}</maven.compiler.source>
         <maven.compiler.target>${java.version}</maven.compiler.target>
         <swagger-annotations-version>1.5.24</swagger-annotations-version>
@@ -308,6 +308,6 @@
         <javax-annotation-version>1.3.2</javax-annotation-version>
         <junit-version>4.13.1</junit-version>
         <maven-plugin-version>1.0.0</maven-plugin-version>
-        <oltu-version>1.0.1</oltu-version>
+        <scribejava-version>8.0.0</scribejava-version>
     </properties>
 </project>