[Feature request] Please support choose floatwindow

[fgheng]

Hello, Please support choose float window, thank you very much

[s1n7ax]

I tried to do this in one of my own plugins. floating window open is async and user input is not. Since vim.api does not provide a callback, the obvious solutions was to use vim.schedule to add user input to event loop. However, picker API needs to be changed to take a callback to pass the window id. I don't know is there is any way to wait mechanism