Pass parameters to external data loader plugins

[tfoldi]

Is your feature request related to a problem? Please describe.
I’m working with large E57 files, where a single file can contain multiple scans, each with millions of points. Loading the entire file into rerun just to visualize or debug a small subset is not feasible due to memory and performance constraints. Currently, rerun loads all points from the file via external data loader plugin, which significantly slows down workflows with large datasets.

Describe the solution you'd like
I’d like rerun’s external data loader plugins to support parameters that allow passing inputs, filters or other information to the plugin. In my case, this could be in the form of scan indices, IDs, or metadata filters passed into the loader, enabling partial loading of only the necessary scans.

Describe alternatives you've considered

• Splitting E57 files manually into separate scans, but this adds preprocessing complexity and increases storage usage.
• Passing environmental variables to select scans like RERUN_E57_DISPLAY_SCANS=0,1,5,10 rerun ../../pointcloud/engine_room.e57. It works from command line, but not a real solution.