Ensure timer runs awaitable

[rodja]

The code

async def update_user():
    await asyncio.sleep(1)

ui.timer(1, lambda: update_user())

Produces the warning Enable tracemalloc to get traceback where the object was allocated.. While the reproduction is a bit artificial, it is beneficial to define lambda statements which return an awaitable because it simplifies passing parameters. We did something similar for button click handlers. This PR adds the same behaviour to ui.timer.