Support for arbitrary $PATH data-loading plugins

[teh-cmc]

Users should be able to put executables on their $PATH that act as arbitrary DataLoaders when opening files with the viewer.

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• Requires Pipe .rrd via stdin/stdout #4494
• Requires Streamline file & file contents loading behind a trait + registry #4495

[teh-cmc]

Fixed at some point in the DataLoader PR series:

• 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
• DataLoaders 6: first-class support for Incompatible #4565
• DataLoaders 7: support for custom DataLoaders #4566
• DataLoaders 8: docs, guide, etc #4567