Support XDG Base Directory Specification for configuration files

[izvyk]

Description

I suggest extending the genericcli/context module to support the XDG Base Directory Specification.

Currently, configuration files are loaded from a hardcoded directory in the user's home if no --config flag is given (e.g., ~/.metalctl/config.yaml). To align with modern Linux standards and keep home directories clean, the config loader should prioritize the following lookup order:

1. XDG config path: $XDG_CONFIG_HOME/<app_name>/config.yaml
   (defaults to ~/.config/<app_name>/config.yaml if the environment variable is unset).

2. Legacy path: ~/.<app_name>/config.yaml
   (for backward compatibility).

This change ensures compliance with the standard while keeping existing setups functional.

Note: needs #194 to be merged first.