Update utils classes according to convention

Signed-off-by: Moti Asayag <masayag@redhat.com>

parodos-model-api/src/main/java/com/redhat/parodos/workflow/utils/CredUtils.java

@@ -2,7 +2,10 @@
 
 import java.util.Base64;
 
-public class CredUtils {
+public abstract class CredUtils {
+
+	private CredUtils() {
+	}
 
 	/**
 	 * Generates a Base64 encoding of username:password string

parodos-model-api/src/main/java/com/redhat/parodos/workflow/utils/WorkContextUtils.java

@@ -33,7 +33,7 @@
  * @author Richard Wang (Github: richardW98)
  */
 
-public class WorkContextUtils {
+public abstract class WorkContextUtils {
 
 	private WorkContextUtils() {
 	}

sdk-utils/src/main/java/com/redhat/parodos/sdkutils/SdkUtils.java

@@ -28,7 +28,10 @@
  */
 
 @Slf4j
-public final class SdkUtils {
+public abstract class SdkUtils {
+
+	private SdkUtils() {
+	}
 
 	/**
 	 * Executes a @see FuncExecutor. Waits at most 60 seconds for a successful result of

workflow-examples/src/main/java/com/redhat/parodos/examples/utils/RestUtils.java

@@ -13,7 +13,7 @@
  * RestUtils is a utility class. All its methods must be declared as static so they can't
  * be overridden.
  */
-public final class RestUtils {
+public abstract class RestUtils {
 
 	/**
 	 * The constructor is private so it'll prevent instantiation.

workflow-service/src/main/java/com/redhat/parodos/security/SecurityUtils.java

@@ -29,7 +29,7 @@
  */
 
 @Slf4j
-public class SecurityUtils {
+public abstract class SecurityUtils {
 
 	private SecurityUtils() {
 	}