Expose `ZarrFormat` in public API

[dstansby]

Currently ZarrFormat isn't exposed in the public API - it would be nice to do this so others can use it. Off the top of my head I propose adding a new sub-module, zarr.types, and importing into there from zarr.core, but there might be a better place to put it in the public API.

[d-v-b]

zarr.types isn't bad, but it might also imply that all our types are defined there, which they currently are not. It would be interesting to have all the types in one place though!

[d-v-b]

see #3355