Allow to customize how SslMasterKeyHandler is enabled (#10513)

Motivation:

Right now after a SslMasterKeyHandler is added to the pipeline,
it also needs to be enabled via a system property explicitly. In
some environments where the handler is conditionally added to
the pipeline this is redundant and a bit confusing.

Modifications:

This changeset keeps the default behavior, but allows child
implementations to tweak the way on how it detects that it
should actually handle the event when it is being raised.

Also, removed a private static that is not used in the wireshark
handler.

Result:

Child implementations can be more flexible in deciding when
and how the handler should perform its work (without changing
any of the defaults).

handler/src/main/java/io/netty/handler/ssl/SslMasterKeyHandler.java

@@ -62,7 +62,7 @@ public abstract class SslMasterKeyHandler extends ChannelInboundHandlerAdapter {
     private static final Throwable UNAVAILABILITY_CAUSE;
 
     static {
-        Throwable cause = null;
+        Throwable cause;
         Class<?> clazz = null;
         Field field = null;
         try {
@@ -120,10 +120,7 @@ public abstract class SslMasterKeyHandler extends ChannelInboundHandlerAdapter {
     @Override
     public final void userEventTriggered(ChannelHandlerContext ctx, Object evt) {
         //only try to log the session info if the ssl handshake has successfully completed.
-        if (evt == SslHandshakeCompletionEvent.SUCCESS) {
+        if (evt == SslHandshakeCompletionEvent.SUCCESS && masterKeyHandlerEnabled()) {
-            boolean shouldHandle = SystemPropertyUtil.getBoolean(SYSTEM_PROP_KEY, false);
-
-            if (shouldHandle) {
             final SslHandler handler = ctx.pipeline().get(SslHandler.class);
             final SSLEngine engine = handler.engine();
             final SSLSession sslSession = engine.getSession();
@@ -143,11 +140,21 @@ public abstract class SslMasterKeyHandler extends ChannelInboundHandlerAdapter {
                 accept(secretKey, sslSession);
             }
         }
-        }
 
         ctx.fireUserEventTriggered(evt);
     }
 
+    /**
+     * Checks if the handler is set up to actually handle/accept the event.
+     * By default the {@link #SYSTEM_PROP_KEY} property is checked, but any implementations of this class are
+     * free to override if they have different mechanisms of checking.
+     *
+     * @return true if it should handle, false otherwise.
+     */
+    protected boolean masterKeyHandlerEnabled() {
+        return SystemPropertyUtil.getBoolean(SYSTEM_PROP_KEY, false);
+    }
+
     /**
      * Create a {@link WiresharkSslMasterKeyHandler} instance.
      * This TLS master key handler logs the master key and session-id in a format
@@ -171,8 +178,6 @@ public abstract class SslMasterKeyHandler extends ChannelInboundHandlerAdapter {
         private static final InternalLogger wireshark_logger =
                 InternalLoggerFactory.getInstance("io.netty.wireshark");
 
-        private static final char[] hexCode = "0123456789ABCDEF".toCharArray();
-
         @Override
         protected void accept(SecretKey masterKey, SSLSession session) {
             if (masterKey.getEncoded().length != 48) {