Use new lib as standard configuration file manager

Have a look on: https://github.com/HBNetwork/python-decouple?tab=readme-ov-file#1-how-to-specify-the-env-path

It will allow Glances to:

• use both environnement variable and Glances configuration file (in that order).
• cast: https://github.com/HBNetwork/python-decouple?tab=readme-ov-file#understanding-the-cast-argument
• use list: https://github.com/HBNetwork/python-decouple?tab=readme-ov-file#built-in-csv-helper

Possible breaking change to be taken into account.