Allow for session "on flush" and "on flushed" callbacks to be async

[schloerke]

Pro:

• You can call session.send_custom_message() which is an await-able method in an "on flush" callback.

Cons:

• The callbacks are not done within the same tick if there is a truly await-able method.
• The next callback is started after the previous callback finishes, which could be a while.
  • Is there a way to "promise all" and launch all callbacks within the same start tick?

[schloerke]

Related: #680 In toggle_switch(), it wants to send a custom message to update the value. (TS implementation does not support session.send_input_message() as rstudio/shiny input switch does not know how to toggle.). The "send custom message" is implemented as async and we do not have a way to start and async method and forget about it. This makes the requires the callback to be async which is currently not supported when registering a "session on flush" callback.

To follow pattern, "session on flushed" callbacks should also be async