Fix: Allow passing in absolute URL for file= parameter

[hybridherbst]

Description of Change(s)

This PR adds support for viewing external files with the demo viewer. It will allow URLs like this to function:

• https://usd-viewer.glitch.me/?file=https://cdn.glitch.global/bee386a1-31e6-4710-8850-a1d5b7026a09/speeder.usdz?v=1646512428693&cameraX=20&cameraY=10&cameraZ=20
• https://usd-viewer.glitch.me/?file=spherebot2.usdz

which makes it useful as a quick way to preview files.

Fixes Issue(s)

Unable to quickly view other files and test the viewer's capabilities.

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[kaischroeder]

Awesome. It's unfortunate that CORS prevents us from loading from arbitrary URLs - such as https://developer.apple.com/augmented-reality/quick-look/models/biplane/toy_biplane.usdz

[hybridherbst]

Yep, for these it will need to be downloaded from the source and then drag-dropped in.

Alternative (but out of scope for a test viewer such as here) would be to fetch them on a server (e.g. a Glitch instance) and then forward them.