Refactor storage

[rabernat]

The goal of this refactor is to remove code from recipes/xarray_zarr.py and add code to storage.py. The aim of this is to improve our abstractions around file handling, such that most of the generic logic related to downloading, caching, and copying files lives in the storage module.

The main new additions are:

• A context manager function called storage.file_opener which takes care of downloading, caching, and copying to local tmpdir
• A new MetadataTarget class which serializes / deserializes data from / to python dictionaries. (We were already doing this in xarray_zarr.py but now it lives in storage
• Better type hints, which caught some edge cases
• Removal of the UninitializedTarget class. I'm sure there was some reason for this at some point, but it seems unnecessary now.

[cisaacstern]

Left a few questions in-line. Some of the internals are still a bit esoteric to my eye, but reading through this was a good chance to take another step towards understanding the inner workings.