Move toBytes() into SASLMechanism

as it's a often needed transformation in the SASL world.

smack-core/src/main/java/org/jivesoftware/smack/sasl/SASLMechanism.java

@@ -244,4 +244,8 @@ public abstract class SASLMechanism implements Comparable<SASLMechanism> {
     }
 
     protected abstract SASLMechanism newInstance();
+
+    protected static byte[] toBytes(String string) {
+        return StringUtils.toBytes(string);
+    }
 }

smack-sasl-provided/src/main/java/org/jivesoftware/smack/sasl/provided/SASLDigestMD5Mechanism.java

@@ -200,7 +200,4 @@ public class SASLDigestMD5Mechanism extends SASLMechanism {
         return responseValue;
     }
 
-    private static byte[] toBytes(String string) {
-        return StringUtils.toBytes(string);
-    }
 }