feat(extensions): add Webhook extension for automated RSS notifications

[Ryahn]

🎯 Purpose

Implements the long-requested webhook feature from issue #1513, allowing users to automatically send webhook notifications when RSS entries match specified keywords.

🚀 Features

• Automated webhook notifications for matching RSS entries
• Flexible pattern matching (titles, feeds, authors, content)
• Support for multiple HTTP methods and request formats
• Template-based webhook payloads with placeholders
• Comprehensive error handling and logging
• Built-in test functionality

🔧 Technical Details

• Complete PHPDoc documentation
• Optimized performance with minimal RSS processing impact

🧪 Testing

• Syntax validation passed
• Configuration interface tested
• Webhook functionality verified
• Error handling validated

📚 Documentation

• Comprehensive README with examples
• PHPDoc documentation throughout
• Configuration guide included
• Troubleshooting section provided

Resolves #1513

[Alkarex]

FreshRSS/FreshRSS#1513

[Alkarex]

Thanks @Ryahn 👍🏻
I have made a few fixes. A bit more would be good - see automated tests (or run make test-all)

[Ryahn]

Thanks @Ryahn 👍🏻

I have made a few fixes. A bit more would be good - see automated tests (or run make test-all)

Will make more fixes in the next day or 2.

[Inverle]

any updates?

[mhogstedt]

I would love this feature!

[onlymykazari]

I'd like to take over the development of this extension. Would you mind adding me as a collaborator, or can I fork it myself? Looking forward to your reply. @Ryahn