Improve support for "random-access" assets (e.g., ZIP files)

Currently, Bevy assumes that an asset loader loads a whole asset. This means something like a ZIP asset loader would load literally all assets in the ZIP, even if only a single asset in the ZIP is needed. Ideally, it should be possible to load singular assets from something like a ZIP file, rather than the whole ZIP file. I call these "random-access" assets, as the common workflow is probably not to load the whole thing and instead to load the individual pieces that are needed.

Start by tracing Bevy's asset-loader and asset-loading entry points to understand where the current whole-asset assumption is enforced. Define how a ZIP loader would identify and load one requested asset without loading the entire archive, then verify that existing whole-asset loading still works alongside the new random-access behavior.

Written by the indexing model from the issue text.