Implement a "zip" kvstore driver

`zip` is not on the list of [KvStore drivers](https://google.github.io/tensorstore/kvstore/index.html#json-KvStore). It should be similar to [zarr.storage.ZipStore](https://zarr.readthedocs.io/en/stable/api/storage.html#zarr.storage.ZipStore).

The main need for zip store is for use in JavaScript and WebAssembly, where a single file can be passed around as a blob, and a directory is very inconvenient (to say the least).

This was first mentioned in https://github.com/google/tensorstore/issues/36#issuecomment-1294043315 (near the end of discussion).