Feature: add support for invoking Tekton trigger EventListener endpoint

[bitsofinfo]

Might be a nice feature to add a simple way to let the plugin invoke an trigger's EventListener endpoint, field specify the endpoint, another for blob of JSON, maybe interpolate jenkins variables in it. The net result in POSTing to the endoint.

At the end of the day there are many other ways to acheive this in jenkins as its just an HTTP post. But might be a good idea to have baked into the plugin for newcomers.