Add support for koanf

[shanduur]

Currently, configuration is managed only through flags. This makes it harder to support more complex or layered configuration, and less convenient for users who want to manage settings via files or environment variables.

• Integrate koanf in addition to flags, to allow:
• Configuration via (one of) YAML/JSON/TOML file.
• Overriding with environment variables.
• Retaining existing flag support for compatibility.