Config overhaul: migrate to figment

[josecelano]

Parent issue: #401

The figment crate provides some functionalities we want to have, like:

• Overwrite and merge config options from different sources.
• Allow users to use default values and only provide values that they want to overwrite.

Subtasks

• Implement a new configuration mod with Figment.
• Reorganize configuration mods creating new submods for config file sections.
• Introduce versioning for configuration, so that we can make breaking changes to the configuration in the future.
• Replace in production the configuration with the new Figment implementation.
• Use Default trait for config sections (not only root config).
• Allow users not to provide values when defaults are OK for them.