Expose PLY load progress

[slimbuck]

This PR addresses the issue in https://forum.playcanvas.com/t/gsplat-loading-progress/38451

The PLY loader is updated to fire progress events during loading.

Notes:

• in practise the content-length response header is not always correct:
  • it can be missing from the response header entirely if server doesn't provide it, and also in some cors situations
  • the size represents the gzipped size of the resource, while the streaming byte sizes are uncompressed bytes (i.e. readBytes can be larger than totalBytes)