refactor: one array class

[d-v-b]

This PR makes 2 fundamental changes to our top-level Array class:

• it adds Array._runner: Runner to the Array. Runner is a protocol that looks like this:

  class SyncRunner:
      """The default `Runner`. Runs coroutines on Zarr's shared background event
      loop via `sync`.
      """
  
      def run(self, coro: Coroutine[Any, Any, T]) -> T:
          return sync(coro)

  the runner parameter allows a caller to provide their own event loop that the array will use when blocking on the execution of a coroutine. If the user doesn't declare a runner, we use a house default, which is just sync. So if you don't request a different runnner, everything is the same.

• it adds async methods for every sync method. the sync methods use self.runner.run(self.do_thing()) to run

This means the AsyncArray class has no use and can be phased out. NOTE: it is not removed.

The goal here is no breaking changes. Removing the AsyncArray class can happen at its own pace. If you find any breaking changes in this PR, we can fix them.

[codecov]

Codecov Report

❌ Patch coverage is 99.24812% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.52%. Comparing base (fe22910) to head (84f8410).

Files with missing lines	Patch %	Lines
src/zarr/api/synchronous.py	93.33%	1 Missing ⚠️
src/zarr/core/group.py	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4034      +/-   ##
==========================================
- Coverage   93.55%   93.52%   -0.04%     
==========================================
  Files          88       88              
  Lines       11896    12009     +113     
==========================================
+ Hits        11129    11231     +102     
- Misses        767      778      +11     

Files with missing lines	Coverage Δ
src/zarr/core/array.py	97.14% <100.00%> (-0.74%)	⬇️
src/zarr/core/attributes.py	96.15% <100.00%> (ø)
src/zarr/core/sync.py	94.64% <100.00%> (+0.30%)	⬆️
src/zarr/metadata/migrate_v3.py	98.36% <100.00%> (ø)
src/zarr/testing/stateful.py	36.46% <ø> (ø)
src/zarr/api/synchronous.py	92.95% <93.33%> (ø)
src/zarr/core/group.py	95.20% <92.30%> (ø)

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[d-v-b]

the main breaking change I was worried about here is Array.__init__, which previously took a single AsyncArray argument, e.g. Array(my_async_array). This still works on this branch, but it's deprecated. The preferred constructor for Array looks much more user-friendly now -- you pass metadata, a storepath, etc.