Allow local execution of scripts under Custom Actions

As of now, one can define a custom action through a custom action channel, However it requires us to either use a REST API, SSM endpoint or OEC to execute scripts.

But in certain use cases one may need to run a short script for example to auto assign an alert and this action can be applied to an alert matching the action policy. In such use cases having local execution of scripts which can be configured on the web UI will allow removing the over head of maintaining an API, SSM document or a server for OEC.