I'm trying to create an Azure Logic App that broadly does the following:
"workItems" : [ { "id": 118, }, { "id": 119, }, etc ]
118, 119, etc
WebJobs Webhooks are now deprecated. Use Azure Functions Generic Webhooks instead - they have direct integration support with Logic Apps.
One option is to use a WebJob Webhook and do the transformation/filter there. I have an example on GitHub of using this to filter posts to Slack. If you already have a Web/Mobile/API app up and running, it's easy to have a WebJob hosted on it so you don't need additional resources, necessarily.
Your other option, you highlighted. Deploy an API App which will do the xform for you.
If you want to go down the WebJob route and need any help, let me know and I'll be glad to assist.