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/examplefilter.hh

@@ -0,0 +1,46 @@
+#pragma once
+/*
+ * 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.
+ */
+
+#include <maxscale/cppdefs.hh>
+#include <maxscale/filter.hh>
+#include "examplefiltersession.hh"
+
+class ExampleFilter : public maxscale::Filter<ExampleFilter, ExampleFilterSession>
+{
+    // Prevent copy-constructor and assignment operator usage
+    ExampleFilter(const ExampleFilter&);
+    ExampleFilter& operator = (const ExampleFilter&);
+
+public:
+    ~ExampleFilter();
+
+    // Creates a new filter instance
+    static ExampleFilter* create(const char* zName, char** pzOptions, MXS_CONFIG_PARAMETER* ppParams);
+
+    // Creates a new session for this filter
+    ExampleFilterSession* newSession(MXS_SESSION* pSession);
+
+    // Print diagnostics to a DCB
+    void diagnostics(DCB* pDcb) const;
+
+    // Returns JSON form diagnostic data
+    json_t* diagnostics_json() const;
+
+    // Get filter capabilities
+    uint64_t getCapabilities();
+
+private:
+    // Used in the create function
+    ExampleFilter();
+};