feat: add OpenHarmony platform backend

[richerfu]

• Tested on all platforms changed
• Added an entry to the changelog module if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created or updated an example program if it would help users understand this functionality

Add OpenHarmony support, i add a independent crate in ohos-rs group. And add it as dependence for winit. Follow #4330

[cavivie]

@richerfu It's been a year and we're almost done, thanks! @kchibisov

[richerfu]

@kchibisov Hi is it possible to review this PR and give us some fallbacks? Can we add ohos platform support for winit? Thanks :)

[ogoffart]

This is the first out of tree platform that we'd have.
But I think we have a problem on how to proceed here.
We have this chain of dependency:
winit-core ← winit-ohos ← winit
And they (will) depend on eachother with a =0.31.0

So when we will release winit 0.31.0 it will have to be done in that order

1. winit-core, from this repository, and other crates, but not winit
2. you release winit-ohos
3. Now we can release winit from this repo.

I think that's too much coordination to ask. And the whole point of asking for out-of-tree platform was that we didn't want to coordinate :-(

So not sure what's best here.

@kchibisov any idea?

Maybe what need to be done here is to have winit-ohos be something like so:

// winit-ohos/lib.rs

pub use winit::*;  // winit-ohos depends on winit and re-export everything

// But override platform
mod platform {
   mod ohos;
}

Then in their Cargo.toml, people would do

[target.'cfg(target_env = "ohos")'.dependencies]
winit = { package = "winit-ohos", version = "0.31" }

But that would still be an issue for the platform_impl module so that's not a full solution.