Database driver adaptations mysql

[tksilicon]

I implemented MysqlClient based on the discussions of #32. Incorporated as much changes as I could. Take a look at it @manyuanrong. Other clients would be implemented if okay.

[manyuanrong]

I made some modifications on this basis. Mainly follow the following principles:

• Different database drivers are placed under drivers and inherit DsoClient
• For Dso users, you don't need to care about whether it is MysqlClient or other clients, and hide the details.

[manyuanrong]

And the dependency of mysql is placed in drivers/mysql.ts. Because in the future, driver/mysql will use dynamic import to avoid loading these dependencies when I use the postgres database.

[manyuanrong]

@tksilicon You can try to implement other drivers on this basis. Let's at least finish postgres

[tksilicon]

@tksilicon You can try to implement other drivers on this basis. Let's at least finish postgres

Will do.