Fix retrieving files from git sources by overriding fetch()

[nikitalita]

while the previous method of just streaming git located files as base64 works for single-file GLB scenes, it doesn't work if the scenes have external dependencies.

We can fix this by doing two things:

• override fetch() on the window to our own version that communicates with the extension to retrieve git scheme files.
• override loader.manager.setURLModifier to set the proper query data for git scheme files.