Add webhooks

[SaadBazaz]

One of the least-cost and easiest ways for #62 . This way, we can sync data to a cloud-hosted server or a service like Zapier or n8n.

[ErikBjare]

This is not happening, as we do not want to encourage people to use centralized servers for sync.

See: https://docs.activitywatch.net/en/latest/remote-server.html