subpackage for chunk key encodings #4268

I propose spinning out the chunk key encoding logic in a separate package. chunk key encodings on their own have a nice narrow scope (map tuples of ints to strings), and it would be nice to have a simple package where the set of available chunk key encodings is defined. Existing as a package rather than adding new functionality to zarr better serves consumers who want to handle chunk encoding / decoding with their own stack, and thus don't need all of zarrs deps.