feat: allows user to specify which mysql package should be used

[Ginden]

Description of change

In attempt to resolve #8534, I'm cherry-picking this small change.

This is necessary to eventually resolve #8713

This allows user using ormconfig.json to specify connector package.

Pull-Request Checklist

• Code is up-to-date with the master branch
• npm run format to apply prettier formatting
• npm run test passes with this change
• This pull request links relevant issues as Fixes #0000
• There are new or updated unit tests validating the change
• Documentation has been updated to reflect this change
• The new commits follow conventions explained in CONTRIBUTING.md

[pleerock]

I would prefer to have two driver types: "mysql" and "mysql2" in the future (both can extend BaseMysqlDriver, etc) and "mysql" only loads "mysql" and "mysql2" only loads "mysql2". This is a bit breaking, but I think it's okay since majority of users don't use mysql2. Anyway, we can accept it and plan this change in 0.4.0