[FR] Ability to provide different DB credentials for running backups

[rob-baker-ar]

Since this #14897 allows for modification of the backup command via a \Closure, it would be helpful, in order to work around this #12557, to have the ability to provide different user credentials for the database that has the extra permissions required for, in our case, adding the --single-transaction argument to the mysqldump command.

This would allow single transaction db backups on MySQL installs at v8.0.32 or above, while not adding a potentially dangerous permission (either the RELOAD or FLUSH_TABLES privilege that is now required for the --single-transaction command argument) to the core database user that is used in on the front-end and throughout the CMS.

Currently, the backup system writes out a temporary MySQL configuration that contains the db username / password, which makes it non-trivial to change the credentials the command uses.

It is theoretically possible to provide credentials via the command arguments, but convention suggests this is not ideal as the password could appear in plain text in log files / shell histories etc. Although, I suppose this already happens by writing the username / password to the temporary MySQL config file. I would think supplying command line credentials is worse that this though, so don't really want to go down this route.