draft: add Webhook extension [draft]

[lukasMega]

here is a draft PR as I promised there: https://github.com/FreshRSS/FreshRSS/discussions/6480#discussioncomment-10583018 I apologize for my long inactivity, I have no time due to family obligations. I would be very glad if someone could take over and enhance the code to address any issues and ensure it meets the necessary criteria for inclusion in this repository. ❤️

see documentation in README.md: https://github.com/lukasMega/Extensions-FreshRSS-/blob/Webhook-extension-draft/xExtension-Webhook/README.md