Ability to specify a higher level directory containing multiple study.db files

[MushroomHunting]

Feature Request

Currently when using the dashboard, I need to specify the sqlite:////"higher_level_dir/lower_level_dir_1/study.db" file. Loading the local address shows the study as expected.
However, I have multiple lower level directories. e.g. my studies would be in

sqlite:////"higher_level_dir/lower_level_dir_1/study.db"
sqlite:////"higher_level_dir/lower_level_dir_2/study.db"
sqlite:////"higher_level_dir/lower_level_dir_3/study.db"
..
sqlite:////"higher_level_dir/lower_level_dir_K/study.db"

I would like to be able to run

optuna-dashbord sqlite:////"higher_level_dir/"

and have all the studies show up in the first dashboard menu so it's not necessary to run optuna-dashboard multiple times

Perhaps I have missed something in the documentation that allows such a functionality

Thanks again for the awesome library