Window customizability in `bevy_winit`

## What problem does this solve or what need does it fill?

WinitPlugin is not very customizable or reusable. Features have to be explicitly exposed to be able to be used.
For example, setting platform-specific options like [`PlatformSpecificWindowBuilderAttributes`'s `titlebar_hidden` for macOS](https://sourcegraph.com/github.com/rust-windowing/winit@master/-/blob/src/platform/macos.rs?L111) for the `WindowBuilder` is impossible without modifying the crate's source.

## What solution would you like?

Some ideas:
1. Allow the user to pass a custom `Window`.
2. Allow the user to modify the `WindowBuilder`.
3. Allow the user to supply a `Fn` for configuring the window?? (Not sure if this is idiomatic)

## Additional context

Context: https://discord.com/channels/691052431525675048/975077788879052870/975095198050828348