add: validator autodiscovery from pySigma plugins

[0xFustang]

Description

Enhances the validation feature to properly autodiscover and load validators from installed pySigma plugins, matching how backends and pipelines are discovered.

Changes:

• Add --list validators option to display all installed validators
• Improve logging to show discovered validators during validation (debug mode)
• Refactor SigmaValidation class

Usage:

# List installed validators
droid --list validators --rules <path>

# See validators loaded during validation
droid --validate --rules <path> --config-file <config> -d