feat(handler): add slack handler implementation

[selcukcukur]

Overview

Slack handler implementation for the logging system.

• Adds a new handler for sending log records to Slack
• Integrates with the existing handler pipeline
• Enables real-time log notifications in Slack channels

Keep it system-focused, not implementation-detailed.

Goals

Bullet list of intentional outcomes of this PR.

• Introduce Slack-based log output handler
• Support structured log formatting for Slack messages
• Integrate with existing handler execution order
• Enable real-time alerting for critical log levels
• Maintain compatibility with current handler interface

Checklist

• Code follows existing architecture style
• No unnecessary abstraction introduced
• Handlers remain isolated
• Ordering behavior preserved
• Memory safety preserved
• Tests updated (if applicable)