Create an Add Layer widget for the ArcGIS JS API v4

[JohnMDye]

There's a bunch of great widgets in the ArcGIS JS API 4.8. One thing noticeable missing is a widget to search a Portal for content and add it to the current map app. For what seems like forever, developers have had to create custom web apps for each web map app they wanted to provide their users. However, I've run into many cases where we wanted to create a web app for streamlining specific workflows which are to some extent, content agnostic. That is to say, its not for a specific feature layer or whatever. It's for performing a specific workflow and we want the user to be able to search the Portal for content, add it to the app and then do what they like.

Web App Builder has a widget for this and that's a decent solution. I don't understand why there's no Add Layer widget for the JSAPI v4.