Make kerchunk dependency entirely optional

[TomNicholas]

Virtualizarr doesn't need to have an explicit dependency on Kerchunk. Kerchunk is one (well it reads many formats) amongst many readers for creating virtual references, and it is now one of two options for writing virtual references (the other being Icechunk). Issue #78 points this out for the readers, but we could make the entire package only optionally depend on kerchunk.

A more urgent reason to change this is Kerchunk does not currently support zarr-python v3 (fsspec/kerchunk#516), which is preventing us ensuring that the rest of this package works with zarr v3 (#182), which is needed for testing Icechunk compatibility (#256).

Changing the virtualizarr business logic to make kerchunk optional is easy - the hard part is that lots of our tests are more reliant on kerchunk than they need to be. Ideally we would either rewrite those integration tests to start from in-memory references rather than files on disk, or use a non-kerchunk approach to generate references from example HDF5 files (#87). I think the former would be neater, but alternatively once the #87 is merged the latter would be a simple change.

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