Create action for ECA

Would a submodule be best for this? Not sure what ECA requires.

I had a need for this, too, so I wrote this submodule which seems to work pretty well. It could use some review and polish, however.

I'm not sure if it's necessary to pass the $entity to the execute() function, for example, and I'm not sure if this strictly depends on the drupal/node, they were carried over from how I had written this for Business Rules and/or other ECA submodules I was using as inspiration.

Any core action will be picked up by ECA.

I don't have a Slack server to try but the patch seems good?
Maybe a doubt about token substitutions: I'm not sure if ECA will automatically apply substitutions or you need to do it in your action code, but it the latter case, I would allow even the channel and user to be tokenized.