Support registry external function in connector and implement a example in the mysql jdbc connector;
store external function stored in memory;
Support push external function push down to jdbc connector and add mysql connector example.
Co-Author: bryawongsz <bryanwangsz@163.com>
Co-Author: Yize Li <liyizeyouxiang@live.cn>
Co-Author: chenpingzeng <chenpingzeng@huawei.com>
Co-Author: wyy566 <531938832@qq.com>
Raghunandan