External DataLoader support for Web Viewer

[methylDragon]

Currently there is no way to support loading of files into external dataloaders from the WebViewer.

It would be good if there was such support, perhaps via sending data to the Websocket server or the process that is hosting the web viewer, which will then invoke the dataloaders.

Perhaps one or more of the following could be implemented:

• Paired with https://github.com/rerun-io/rerun/issues/5324 , have the WebViewer send the data from the file to the process/Websocket server over any transport to load in the data
• Send the file contents over transport (gRPC?) to the hosting process
• Send the file contents over via the same Websocket connection tied to the server
• Send the file path instead
  • This option is probably not the best because it would require that the web viewer is located in the same host
• Add a new UI to the viewer to specify the URL of a file to load, and send that instead

Finally, maybe it could be possible to load a dataloader executable at runtime to be run in the web viewer, much like how Foxglove extensions could be loaded.

Apologies if some of the suggestions don't make sense, I am not as familiar with the architecture of Rerun.