Allowing for App URL schemes for link buttons

[mayeradelman]

I was wondering if it would be possible to add app link support for URL buttons for when there's no corresponding http endpoint to open the app. I know this exists for the Discord app already, but it would be super helpful if all apps were supported as well. Currently, when trying to send a button with an app URL I get the following error message:

"Scheme "{custom_scheme}" is not supported. Scheme must be one of ('http', 'https', 'discord')."

[Cynosphere]

Duplicate of #6158