improve type hint for OptionalEventCallable

[zrr1999]

Description

The current annotation is only correct for synchronous functions.
I will replace Callable[[ControlEvent], None]] with Callable[[ControlEvent], Union[None, Coroutine[None, None, None]]] to support asynchronous functions.

Test Code

import flet as ft


async def demo(e):
    pass


ft.TextButton("Hello, World!", on_click=demo)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• I signed the CLA.
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings

• New and existing tests pass locally with my changes
• I have made corresponding changes to the documentation (if applicable)

Additional details

[ndonkoHenri]

Thanks for your contribution! You raised an important point.
I made a commit to further enhance the typing of event-handlers in all the files.

I also considered what the author of #3673 said concerning the return type:

flet probably does not care about the return value, and therefore should not place any restrictions on it.