implement `store.list_prefix` and `store._set_dict`

[d-v-b]

• fixes / implements list_prefix for stores. list_prefix(prefix=foo) now consistently returns keys with the shared prefix foo stripped. I'd be fine altering this to return absolute keys instead.
• implements tests for list_prefix on the StoreTests base class
• adds a _set_dict(Mapping[str, Buffer]) method to stores that allows declaring a collection of key: value pairs to write to storage. The primary usage is to make store tests simpler via a declarative API. I also suspect that making tests simpler anticipates making other code simpler. The default implementation of _set_dict simply wraps store.set, but it's easy to imagine fancier batching / transaction implementations.
• alters some of the store tests to use _set_dict.
• adds some missing type annotations in the remotestore tests

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]

@martindurant i'm on flaky train internet and the github web API is giving me inconsistent signals about whether you have been requested for review; to be clear, I would appreciate your review :)

[martindurant]

list_prefix(prefix=foo) now consistently returns keys with the shared prefix foo stripped. I'd be fine altering this to return absolute keys instead.

What matters is how we intend to call it! fsspec likes to provide full paths (but has explicit prefix interfaces) - of course it's fine to tailor the situation to what we need, but I don't really know what that is.

I would appreciate your review

I may have got a notification or a few.