[ISSUE #9760] Add postgres as an alternative external data source

[cloorc]

Please do not create a Pull Request without creating an issue first.

What is the purpose of the change

Add postgres as an alternative external data source support besides mysql.

Brief changelog

1. Append new data source constant after DataSourceConstant#MYSQL;
2. Add new postgres mapper implementation;
3. Add UNIT-tests about this external data source implementation;
4. Add an integration of docker image in module nacos-distribution(optional, but mostly enough for deployment);

Verifying this change

Fix #9760 #9499

[CLAassistant]

All committers have signed the CLA.

[wuchubuzai2018]

你这个应该会被关闭掉，官方不允许提交数据库插件到主仓库

[cloorc]

你这个应该会被关闭掉，官方不允许提交数据库插件到主仓库

Sorry, I don't know this policy. Why should be like this?

[KomachiSion]

Please submit plugin to https://github.com/nacos-group/nacos-plugin

[KomachiSion]

There are some one has submit Postgresql plugin nacos-group/nacos-plugin#18

[dylan-tao]

目前我这边已经开源支持postgresql的nacos-2.2.0版本（已投产），可以直接食用：https://github.com/dylan-tao/nacos-gaussdb