Or instead of using v5 use Helix:
There already is:
You can long poll on the topic you would of subscribed to on Webhooks under the helix API.
See Reference | Twitch Developers
GET https://api.twitch.tv/helix/users/follows?to_id=[user ID]
Webhooks require for the application that is receiving to be accessible from the outside internet inbound to the application. That generally doesn’t happen with a desktop/mobile application. So you will have to long poll. Or implement the server that you don’t want to implement to relay.