Add support for creating users with the caching_sha2_password auth plugin

[fulghum]

This change enables customers to create users configured to authenticate with the caching_sha2_password auth plugin. The generated authentication string uses the same logic as MySQL's caching_sha2_password auth plugin. Users created with caching_sha2_password can not yet authenticate with a GMS server – the next change in this series will enable that.

Example usage:

CREATE USER fred@localhost identified with caching_sha2_password by 'pa$$w0rd';

Depends on: dolthub/vitess#387

Related to: dolthub/dolt#8496

[zachmu]

This looks fine, but do we have any top-level integration tests that validate creating a user with a certain password and then connecting with it?

[fulghum]

This looks fine, but do we have any top-level integration tests that validate creating a user with a certain password and then connecting with it?

Thanks! Yup – I'm working on those tests currently. This change enables creating users for caching_sha2_password auth, but they still can't log in yet. The next PR will include the changes to enable the caching_sha2_password auth method in our auth server, and that PR will include the tests to connect to the server with caching_sha2_password configured accounts. That will be enough for people to opt-in to using caching_sha2_password, but we'll still keep mysql_native_password as our default for a bit longer to make sure we get the kinks worked out.