Allow the use of a `fsspec` filesystem

As it says in the title!

IIUC `zict.File` only reads/writes to a local filesystem. It would be great to instead be able to write to any filesystem supported by `fsspec`.

Start by locating the zict.File implementation and its existing filesystem tests, then inspect how it currently opens local paths. Define the supported fsspec filesystem interface from the project’s conventions and add coverage showing that reads and writes work through it; done means File can use an fsspec-supported filesystem rather than only local storage.

Written by the indexing model from the issue text.