WIP: Support extern context impl

[sagudev]

Connections
Based on #6619, fixes #6330

Description
This PR enables users of wgpu to provide custom Instance impl (and other wgpu types as all other types are derived from Instance->Adapter->Device), by adding Custom variant and custom backend that has dyn wrapper for all types: struct DynDevice(Arc<dyn DeviceInterface>). I'm intermixing Custom with Dyn because I cannot really decide on name (CustomDevice is weird, but we already have Dyn* in wgpu-hal).
There are few rough sports:

• RenderBundleEncoderInterface::finish (other backends hacks around by not using dyn dispatch, but we can't:

  wgpu/wgpu/src/api/render_bundle_encoder.rs

  Lines 54 to 60 in b876b8c

  	pub fn finish(self, desc: &RenderBundleDescriptor<'_>) -> RenderBundle {
  	let bundle = match self.inner {
  	#[cfg(wgpu_core)]
  	dispatch::DispatchRenderBundleEncoder::Core(b) => b.finish(desc),
  	#[cfg(webgpu)]
  	dispatch::DispatchRenderBundleEncoder::WebGPU(b) => b.finish(desc),
  	};

• InstanceInterface::new cannot work because we cannot use dyn dispatch to create new instance (instance must be created by consumer)
• webgpu only methods are unimplemented!

future work:

• allow wgpu on native to be without core (libraries using wgpu (ex: vello) usually get instance/device from their users, so they would want to disable core if user is providing instance anyway), although this make wgpu to play 2 roles: abstraction provider and impl provider (of core and webgpu backend). Should we somehow split this into two creates? (so libraries would only depend on wgpu-abstract while consumers of those libraries would use wgpu-provider that would also depend on wgpu-abstract and toggle appropriate features for enums: core,webgpu,custom)

Testing
For testing we simply pass wgpu-core/webgpu as dyn Instance (be5acfa), so this works, but I plan to do example of creating dummy interface (for testing purposes).

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy. If applicable, add:
  • --target wasm32-unknown-unknown
  • --target wasm32-unknown-emscripten
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.

[sagudev]

TBH, I have no idea what's wrong with WASM.

[sagudev]

This is triggering visibility problems for WASM, but I do not know why.

[cwfitzgerald]

Ping me if this needs looking at or you need help before it is out of draft, going to un-assign myself for bookeeeping