Use StringDType when reading from zarr string variables

[jsignell]

• Closes Zarr chunks ignored when reading arrays with StringDType #11054
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst

Down to 4 failures locally which all seem to have special handling for zarr_3 so should be quick to fix

FAILED xarray/tests/test_backends.py::TestZarrNoConsolidatedMetadataSupport::test_roundtrip_empty_vlen_string_array[3] - AssertionError: assert False
FAILED xarray/tests/test_backends.py::TestZarrDirectoryStore::test_roundtrip_empty_vlen_string_array[3] - AssertionError: assert False
FAILED xarray/tests/test_backends.py::TestZarrWriteEmpty::test_roundtrip_empty_vlen_string_array[3] - AssertionError: assert False
FAILED xarray/tests/test_backends.py::TestZarrDictStore::test_roundtrip_empty_vlen_string_array[3] - AssertionError: assert False

[jsignell]

@dcherian right as you were approving I was changing the logic to handle misc na_object values. Still ✅ ?

[dcherian]

LGTM!