Add C++ filter example

The filter example uses the C++ template which helps new filter creation
by showing the minimal implementation required for a filter.

examples/examplefiltersession.cc

@@ -0,0 +1,43 @@
+/*
+ * Copyright (c) 2016 MariaDB Corporation Ab
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file and at www.mariadb.com/bsl11.
+ *
+ * Change Date: 2020-01-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2 or later of the General
+ * Public License.
+ */
+
+// All log messages from this module are prefixed with this
+#define MXS_MODULE_NAME "examplefilter"
+
+#include "examplefiltersession.hh"
+#include "examplefilter.hh"
+
+ExampleFilterSession::ExampleFilterSession(MXS_SESSION* pSession)
+    : mxs::FilterSession(pSession)
+{
+}
+
+ExampleFilterSession::~ExampleFilterSession()
+{
+}
+
+//static
+ExampleFilterSession* ExampleFilterSession::create(MXS_SESSION* pSession, const ExampleFilter* pFilter)
+{
+    return new ExampleFilterSession(pSession);
+}
+
+int ExampleFilterSession::routeQuery(GWBUF* pPacket)
+{
+    return mxs::FilterSession::routeQuery(pPacket);
+}
+
+int ExampleFilterSession::clientReply(GWBUF* pPacket)
+{
+    return mxs::FilterSession::clientReply(pPacket);
+}