Support Tor onion services as webhook targets

[JeremyRand]

Description

It would be useful for webhooks to support connecting to Tor onion services. This could be implemented by checking for the .onion TLD, and routing the webhook connection through a Tor SOCKS proxy if the webhook target is on the .onion TLD.

Context

I'd like to try using a webhook as suggested here, but the infrastructure that the webhook would connect to does not have a publicly accessible IP address for security/privacy reasons. A Tor onion service is an easy and secure way to allow incoming connections for that infrastructure without exposing a public IP address.