Change MessageFormatter / FormattingTuple to public (netty#12131)

Motivation:

Allow other Logging integration to use optimised MessageFormatter implementation

Modifications:

Change MessageFormatter /  FormattingTuple to public  

Result:

Fix netty#12129

common/src/main/java/io/netty/util/internal/logging/FormattingTuple.java

@@ -42,7 +42,7 @@
 /**
  * Holds the results of formatting done by {@link MessageFormatter}.
  */
-final class FormattingTuple {
+public final class FormattingTuple {
 
     private final String message;
     private final Throwable throwable;

common/src/main/java/io/netty/util/internal/logging/MessageFormatter.java

@@ -108,7 +108,7 @@
  * {@link #format(String, Object, Object)} and
  * {@link #arrayFormat(String, Object[])} methods for more details.
  */
-final class MessageFormatter {
+public final class MessageFormatter {
     private static final String DELIM_STR = "{}";
     private static final char ESCAPE_CHAR = '\\';