Add changelog entry for external texture support (#8123)

Author: jamienicol

CHANGELOG.md

@@ -59,6 +59,8 @@ By @Vecvec in [#7913](https://github.com/gfx-rs/wgpu/pull/7913).
 
 - Added mesh shader support to `wgpu`, with examples. Requires passthrough. By @SupaMaggie70Incorporated in [#7345](https://github.com/gfx-rs/wgpu/pull/7345).
 
+- Added support for external textures based on WebGPU's [`GPUExternalTexture`](https://www.w3.org/TR/webgpu/#gpuexternaltexture). These allow shaders to transparently operate on potentially multiplanar source texture data in either RGB or YCbCr formats via WGSL's `texture_external` type. This is gated behind the `Features::EXTERNAL_TEXTURE` feature, which is currently only supported on DX12. By @jamienicol in [#4386](https://github.com/gfx-rs/wgpu/issues/4386).
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 ### Changes
 
 #### General