Add support for sha256_password pluggable authentication

[AlekSi]

https://dev.mysql.com/doc/refman/8.0/en/sha256-pluggable-authentication.html

Edit: Note that support for caching_sha2_password is already implemented. See #794.

[elemount]

It seems that is related on #552 , and the implement is depends on it will be accepted or not.

[twocode]

Previously I made a patch supporting auth switching to native password (sha1) on this and I definitely vote that we should start sha256 support now.

[geertjanvdk]

Just a friendly note that with MySQL 8.0.4-rc comes a new default for authentication: caching_sha2_password is now used when creating users.

If you want to use a go-sql-driver/mysql supported authentication plugin, you have to create/alter users to use a different one.

As as workaround, you could use the MySQL Native Password plugin:

ALTER USER  'youruser'@'%' IDENTIFIED WITH 'mysql_native_password' BY 'secret';

Note that you will need to explicitly set "allowNativePasswords=True" in the DSN.

(I did implement auth plugins in MySQL Connector/Python, I know the pain)

[lefred]

Hello, if you are looking for help, MySQL engineers working on authentication plugins and connectors will be happy to assist you. You can reach us on the MySQL Community Slack (http://lefred.be/mysql-community-on-slack/). Keep the good work !
Cheers,