WasmFS: XHR backend

[tlively]

This was noted as a possibility in some of our WasmFS design docs and an internal Google user has reached out to me with a use case for an XHR backend, so it would be nice to have.

[mmarczell-graphisoft]

Would this download the remote file every time it is accessed or cache it after the first time?

We implemented something like this - albeit at a higher layer - where the first time the app tries to read a file, it is downloaded, saved to a Blob, which subsequent reads are served from. It would be great if that functionality could be implemented inside Emscripten or added as a plugin.

[mere-human]

How is it different from fetch backend?