Support hot-reloading for a const HandleUntyped handle to an asset

[superdump]

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

I would like to make the core shaders hot-reloadable, behind a feature. So by default bevy would use include_str!() but it could optionally use something like asset_server.load() instead.

What solution would you like?

I want to be able to associate something like:

pub const PBR_SHADER_HANDLE: HandleUntyped =
    HandleUntyped::weak_from_u64(Shader::TYPE_UUID, 4805239651767701046);

with an asset loaded and watched by the AssetServer, so the PBR_SHADER_HANDLE can be used as the handle, but so that the asset server file watcher will watch for changes and notify PBR_SHADER_HANDLE when the asset was changed.

Additional context

• Hot-reloading works by notifying the HandleId that can be created from the AssetPath (via the AssetPathId). This means that only the handle created from the AssetPath can be notified.
• There can only be one handle to an asset. I have been trying to make it so that there can be multiple handles, so that the const HandleUntyped can also be associated with the asset, and will be notified of changes but this seems to be way more difficult than expected.
  • Note that loading the asset and then using asset_server.set_untracked() to set the HandleUntyped for the asset will not support hot-reloading because only the HandleId calculated from the AssetPath will be notified.

[superdump]

I made a PR as an attempt at this: #3673