Support GitHub webhook secrets

[konklone]

GitHub webhooks can have a shared secret, that the webhook can use to verify that the payload came from GitHub.

It doesn't look like this is supported here, but I think it should be!

[NotBobTheBuilder]

👍

[karvapallo]

Yes, this should definitely be implemented.

[karvapallo]

I made a fork with support for Github webhook secrets, and you can get it from here:

https://github.com/karvapallo/webhook-deployer

I made a pull request back to this repo for that feature.