Part1: MDEV-20837 Add MariaDB_FUNCTION_PLUGIN

- Defining MariaDB_FUNCTION_PLUGIN
- Changing the code in /plugins/type_inet/ and /plugins/type_test/
  to use MariaDB_FUNCTION_PLUGIN instead of MariaDB_FUNCTION_COLLECTION_PLUGIN.
- Changing maturity for the INET6 data type plugin from experimental to alpha.

include/mysql/plugin_function.h

@@ -0,0 +1,54 @@
+#ifndef MARIADB_PLUGIN_FUNCTION_INCLUDED
+#define MARIADB_PLUGIN_FUNCTION_INCLUDED
+/* Copyright (C) 2019, Alexander Barkov and MariaDB
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; version 2 of the License.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, write to the Free Software
+   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1335  USA */
+
+/**
+  @file
+
+  Function Plugin API.
+
+  This file defines the API for server plugins that manage functions.
+*/
+
+#ifdef __cplusplus
+
+#include <mysql/plugin.h>
+
+/*
+  API for function plugins. (MariaDB_FUNCTION_PLUGIN)
+*/
+#define MariaDB_FUNCTION_INTERFACE_VERSION (MYSQL_VERSION_ID << 8)
+
+
+class Plugin_function
+{
+  int m_interface_version;
+  Create_func *m_builder;
+public:
+  Plugin_function(Create_func *builder)
+   :m_interface_version(MariaDB_FUNCTION_INTERFACE_VERSION),
+    m_builder(builder)
+  { }
+  Create_func *create_func()
+  {
+    return m_builder;
+  }
+};
+
+
+#endif /* __cplusplus */
+
+#endif /* MARIADB_PLUGIN_FUNCTION_INCLUDED */