Publicly expose icechunk's `validate_containers` kwarg

[TomNicholas]

Icechunk now has a validate_containers kwarg to .set_virtual_ref/.set_virtual_refs. We should publicly expose this in .virtualize.to_icechunk().

I think we want to make True the default and expose it for overriding. The reason for that is normally you want to check this up front (so at write time instead of read time), but sometimes you want to just dump references, e.g. because someone might want to get some pre-made kerchunk references and dump them into icechunk without ever setting up a virtual chunk container.

[TomNicholas]

Will now be closed by fe7f817 of #774