I have a web application that provides APIs to be invoked. In my use case, I would like to invoke certain APIs when opening and closing a code review. However I discovered that Herald Rules does not support custom actions. It would be nice if the task could be running some PHP or shell script.
I hope Herald Rules is the right place for this type of use case to go as I was trying to make a plug in for Phabricator but realized that there is no way to listen to this type of updates.
We have lots of APIs for external things, but without knowing what you're trying to do specifically, we can't really give you advice here. Herald / Mail are absolutely never the correct answer. Doorkeeper, Harbormaster, Feed Hooks, all available out of the box.