Protocol for event handlers is too restrictive

### Describe the bug

The `BackgroundTask` protocol only defines support for normal functions and methods. The code, however, allows background tasks to be a `NativeHandler`, coroutine, or generator.

```Python
class BackgroundTask(Protocol):
def __call__(self, app: App, **kwargs: Any) -> None: ...
```

### Steps to reproduce

Run `mypy --strict` against the app below:

```python
import asyncio
from typing import Any

import toga

class AutoExit(toga.App):

async def exit_soon(self, app: toga.App, **kwargs: Any) -> None:
await asyncio.sleep(3)
print(">>> successfully started...exiting <<<")
self.exit()

def startup(self) -> None:
"""Show the Toga application."""
self.add_background_task(self.exit_soon)

main_box = toga.Box()

self.main_window = toga.MainWindow(title=self.formal_name)
self.main_window.content = main_box
self.main_window.show()

def main() -> AutoExit:
return AutoExit()
```

```console
❯ mypy --strict src/plugintesting/app.py
src/plugintesting/app.py:19: error: Argument 1 to "add_background_task" of "App" has incompatible type "Callable[[App, KwArg(Any)], Coroutine[Any, Any, None]]"; expected "BackgroundTask" [arg-type]
Found 1 error in 1 file (checked 1 source file)
```

### Expected behavior

This protocol accurately represents the objects that can be passed to `add_background_task()`.

Note: Other handler protocols may be affected; I just wanted to document this one while I was seeing it.

### Screenshots

_No response_

### Environment

- Operating System: pop os 22.04
- Python version: 3.10.13
- Software versions:
- Briefcase: `0.3.17.dev45+g8e412970.d20231104`
- Toga: `0.4.0`

### Logs

_No response_

### Additional context

- https://github.com/beeware/toga/issues/2042

Start by locating the BackgroundTask protocol and add_background_task() definition, then inspect how NativeHandler, coroutines, and generators are handled. Update the relevant typing definitions and check the provided mypy --strict example, along with any affected handler protocols; done means supported handlers type-check without weakening unrelated checks.

Written by the indexing model from the issue text.