Configuration overhaul: migrate configuration to Figment

Parent issue: https://github.com/torrust/torrust-index/issues/586
Relates to: https://github.com/torrust/torrust-tracker/issues/850

We migrated from `config` to `figment` crate in the Tracker: https://github.com/torrust/torrust-tracker/issues/850.

I think we could apply the same changes here.