Add OpenProject integration

[mateuszmandera]

OpenProject is an open source project management system. They have some outgoing webhooks that we've received a feature request for integrating.

There's an old issue for this at #26445 but it's mostly just a question from a contributor attempting to add the integration rather than an issue for tracking the feature itself. It has a little bit of background info that might be helpful, but I'm closing it in favor of this.

[zulipbot]

Hello @zulip/server-integrations members, this issue was labeled with the "area: integrations" label, so you may want to check it out!