support custom action plugins

[mottibec]

I started looking into this for a project I'm working on.

just wondering how to implement this,

I thought that in case the request is a custom action we would want to have something like this in Core.Execute

if (isCustomRequest) {
  response = OrganizationResponse { Results = pluginContext.OutputParameters };
}

isCustomRequest can be based on the baseType of request that is OrganizationRequest and/or not in Mappings.RequestToEventOperation or RequestHandlers or any other data collection that would hold the list of system actions.

any suggestions, comments or thoughts?