DataLoaders 5: add support for external binary DataLoaders (PATH)

[teh-cmc]

Turn any executable in your PATH into a DataLoader by logging data to stdout.

Includes examples for all supported languages.

Checks:

• rerun-loader-rust-file just prints help message
• rerun-loader-rust-file examples/rust/external_data_loader/src/main.rs successfully does nothing
• rerun-loader-rust-file examples/rust/external_data_loader/src/main.rs | rerun - works
• rerun-loader-python-file just prints help message
• rerun-loader-python-file examples/python/external_data_loader/main.py successfully does nothing
• rerun-loader-python-file examples/python/external_data_loader/main.py | rerun - works
• rerun-loader-cpp-file just prints help message
• rerun-loader-cpp-file examples/cpp/external_data_loader/main.cpp successfully does nothing
• rerun-loader-cpp-file examples/cpp/external_data_loader/main.cpp | rerun - works
• cargo r -p rerun-cli --no-default-features --features native_viewer -- examples/assets
• Native: File > Open > examples/assets/
• Native: 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

• PR Build Summary
• Docs preview
• Examples preview
• Recent benchmark results
• Wasm size tracking

[github-actions]

Size changes

Name	cmc/dataloaders_4_folders	4521/merge	Change
detect_and_track_objects.rrd	(none)	55.21 MiB	+100%
plots.rrd	204.80 kiB	194.56 kiB	-5.00%

[teh-cmc]

I forgot to update the DataLoader docstring to link the new loader, will do so in the docs/howto guide PR.

[emilk]

You should document somewhere what we expect from a loader, eg in a //!-level docstring

In particular:

• what arguments to expect (--recording-id)
• what exit code to use for unsupported file types
• what and where to output (.rrd data to stdout)

This should also be added as a how-to guide on rerun.io/docs!

[teh-cmc]

I have an upcoming PR that adds a web guide, I'll add a link to that guide in the doc string then