Upgrade App's UI function to async

[schloerke]

Would allow for app authors to make async calls within their UI function.

From what I can see,

py-shiny/shiny/_app.py

Line 305 in 3e587ab

ui = self._render_page(self.ui(request), self.lib_prefix)

is the only place it is executed and is already within an async function.

We could wrap it in async at the very beginning, avoiding repeated is async checks.