Support parsing JSON-encoded "relay webhooks" messages

[yargevad]

[abh]

This patch doesn't work (without some extra glue) because the relay_message, uh, messages don't have a "type". I emailed support and was told this was intentional.

[yargevad]

You're right that extra glue is currently needed to parse events sent to relay webhook endpoints, since the current auto-parsing function calls them Unknown afaict.