Add full OAuth2 support to jersey2-experimental codegen (#6183)

* Add full Oauth2 support to Jersey client

* Regenerate jersey2-experimental sample

* Regenerate all java clients

Author: cbornet

modules/openapi-generator/src/main/resources/Java/StringUtil.mustache

@@ -2,6 +2,9 @@
 
 package {{invokerPackage}};
 
+import java.util.Collection;
+import java.util.Iterator;
+
 {{>generatedAnnotation}}
 public class StringUtil {
   /**
@@ -47,4 +50,23 @@ public class StringUtil {
     }
     return out.toString();
   }
+
+  /**
+   * Join a list of strings with the given separator.
+   *
+   * @param list      The list of strings
+   * @param separator The separator
+   * @return the resulting string
+   */
+  public static String join(Collection<String> list, String separator) {
+    Iterator<String> iterator = list.iterator();
+    StringBuilder out = new StringBuilder();
+    if (iterator.hasNext()) {
+      out.append(iterator.next());
+    }
+    while (iterator.hasNext()) {
+      out.append(separator).append(iterator.next());
+    }
+    return out.toString();
+  }
 }