Support loading .ply files

[teh-cmc]

Support loading .ply files with the CLI and Viewer (not the SDKs!).

• Requires Pipe .rrd via stdin/stdout #4494
• Requires Streamline file & file contents loading behind a trait + registry #4495
• Requires Support for arbitrary $PATH data-loading plugins  #4496

[Wumpf]

ply is generally a mesh/triangle format (it's literally the Stanford Triangle Format). However, it's worth noting that a lot of people also use it for point clouds! We need to reconciliate this somehow

[teh-cmc]

I'm only focusing on point clouds for the builtin loader for now, at least as a start (point cloud plys are a very common user request).

After which we can:

• expand on that builtin loader in the future to make it configurable etc, or
• just let users add their own ply loader now :)

[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