[metal] Use raw-window-metal to do layer creation

[madsmtm]

You know, all that code and documentation I just wrote in #6107? Let's throw it away into a library, so that wgpu, ash, vulkano, i-slint-renderer-skia and so on can all benefit from it.

raw-window-metal is that library (existed previously, I've updated it in rust-windowing/raw-window-metal#19).

This makes Wgpu depend transitively on objc2, so the concerns about doing that apply here as well, see also #5641.

Description
Use the raw-window-metal crate to do layer creation. This uses much the same approach as we do currently, except that it also uses observers internally, which fixes resizing on high DPI screens when using Wgpu together with an NSView using auto-layout.

Though it probably won't matter much, it's also more reliable to update the scale factor this way, rather than haphazardly in configure.

The logic in raw-window-metal could be re-implemented in Wgpu to avoid the dependency, however I deemed it complex enough that I thought it wiser to centralize the implementation in one place.

Testing
See #6107 (comment).

Checklist

• Run cargo fmt.
• Run cargo clippy.
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.
  • I did not deem this to be necessary if the change is released in the same release as [metal] Improve layer initialization and resizing #6107.