[Suggestion] Allow the user to pick a config file outside the presets folder

[ejektaflex]

Use case: I keep all of my training data in git. As I iterate on new versions of a LoRa, I branch off of successful versions and continue iterating. If I want to see what I did to make, say, version 39 of the Lora so much better than version 35, I can just diff the commit changes to find out.

With Kohya, everything is loaded from a config file that you can choose. As such, it works with git really well. However, OneTrainer seems to only allow loading configs from the training_presets folder. This means that if I want to keep my changes in git, I need to copy my config file out of the presets folder every single time I want to commit my changes and keep track of what I've learned.