Feat/latency store

[d-v-b]

This PR adds a new store class, WrapperStore, that implements Store but wraps an existing store instance and delegates all of its methods to those of the wrapped store. I then use WrapperStore to define a store class that exists for performance testing, called LatencyStore, which (at the moment) adds parametric latency to the set and get methods.

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/tutorial.rst
• Changes documented in docs/release.rst
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[d-v-b]

anyone interested in implementing n5 in zarr-python 3 might find a use for WrapperStore (cc @dstansby )

[jhamman]

Looks good. Need to sort out the AsyncIterator type question but otherwise this looks great.

[d-v-b]

@jhamman the type annotations and docstrings are ironed out