Extract Root Filesystem Guest-side

[crepererum]

What

Move from host-side root file system population (esp. for Python guests) to guest-side root file system population.

Why

We currently use a rather non-standard interface to pass a TAR file from the guest to the host, which then extracts it:

datafusion-udf-wasm/wit/world.wit

Line 94 in 9e3d58a

root-fs-tar: func() -> option<list<u8>>;

It would be nicer if we just give the guest a read/write virtual file system so it can do that that itself. This also provides more flexibility regarding how the file system is stored (technically the guest could even fetch it from a remote source) and shrinks the possible attack surface.

How

This needs #133 first. Then move the tar extraction from the host to the guest and remove our custom WIT interface.