COMMIT PER COMMIT!

That's the nasty one... there's always a nasty one 😬

Introduces the DataLoader trait, which can load any file, either from a filepath by reading from the local filesystem, or from a pre-loaded file's contents.

DataLoaders are run in parallel where possible, and have complete say over what data they are interested in or not (i.e. they are not registered based on an extension, which is very limited in practice).

I've decided to commit small example assets for the types we support, cause I feel we really need a fast way of checking whether everything still works from time to time.
It's pretty light but might be controversial, I figure.

Checks:

• cargo r -p rerun-cli --no-default-features --features native_viewer -- examples/assets/example.{glb,gltf,obj,jpg,png,rrd}
• Native: File > Open > examples/assets/*
• Native: Drag-n-drop > examples/assets/*
• Web: File > Open > examples/assets/*
• Web: Drag-n-drop > examples/assets/*

Part of a series of PRs to make it possible to load any file from the local filesystem, by any means, on web and native:

• DataLoaders 0: utility for hierarchical EntityPath from file path #4516
• DataLoaders 1: introduce, and migrate to, DataLoaders #4517
• DataLoaders 2: add text-based DataLoader (.txt, .md) #4518
• DataLoaders 3: add 3D point cloud DataLoader (.ply) #4519
• DataLoaders 4: add generic folder DataLoader #4520
• DataLoaders 5: add support for external binary DataLoaders (PATH) #4521
• TODO: register custom loaders
• TODO: high level docs and guides for everything related to loading files

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested the web demo (if applicable):
  • Full build: app.rerun.io
  • Partial build: app.rerun.io - Useful for quick testing when changes do not affect examples in any way
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG

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