Portal Support With Custom UI

**Is your feature request related to a problem? Please describe.**
No

**Describe the solution you'd like**
In the Azure Portal, I would like to have the ability to create a new Management Group Deployment Stack from a Bicep template using a "Deploy to Azure" button that supports a custom UI definition. This will allow me to give my customers an easy and intuitive way to deploy Azure resources into their Tenants. So really, I'm talking about four capabilities (in order of importance):

1. Ability to manage a Management Group level Deployment Stack in the Portal
2. Ability to create a new Deployment Stack directly in the Portal
3. Ability to use a Deploy to Azure button to trigger creation of a new Deployment Stack
4. Ability to use a custom UI definition with the above two capabilities

**Describe alternatives you've considered**
There are no viable alternatives. My customers are often inexperienced with Azure. Asking them to use the Azure CLI to deploy a Deployment Stack leads to a variety of challenges, the biggest one being that they first need to have Azure CLI on their local machine or understand how to use Cloud Shell.

**Additional context**
I know that today, if you do a Deployment you are given the option to instead do a Deployment Stack, but I want the ability for this to happen without additional clicks by the user. I'd also like to understand when this capability is expected to be live because it will impact my own roadmap.