Fix type annotations for ArrayRecordDataSource.

[tillahoffmann]

This PR fixes type annotations for ArrayRecordDataSources. Currently, ArrayRecordDataSource does not conform to the RandomAccessDataSource protocol for two reasons:

1. The stub that indicates ArrayRecordDataSource is not supported on Windows is missing __len__ and __getitem__.
2. The implementation uses record_key: SupportsIndex instead of index: int as the __getitem__ argument. Renaming the argument is backwards-compatible because it is never accessed as a keyword argument in normal usage.

Here's an example.

from grain import MapDataset
from grain.sources import ArrayRecordDataSource


ds = ArrayRecordDataSource([])
MapDataset.source(ds)

Before

$ pyright playground/ards.py
/Users/till/git/grain/playground/ards.py
  /Users/till/git/grain/playground/ards.py:6:19 - error: Argument of type "ArrayRecordDataSource" cannot be assigned to parameter "source" of type "Sequence[T@source] | RandomAccessDataSource[T@source]" in function "source"
    Type "ArrayRecordDataSource" is not assignable to type "Sequence[T@source] | RandomAccessDataSource[T@source]"
      "ArrayRecordDataSource" is incompatible with protocol "RandomAccessDataSource[T@source]"
        "__len__" is not present
          "__getitem__" is an incompatible type
            Type "(record_key: SupportsIndex) -> bytes" is not assignable to type "(index: int) -> T@RandomAccessDataSource"
      "ArrayRecordDataSource" is not assignable to "Sequence[T@source]" (reportArgumentType)
1 error, 0 warnings, 0 informations

After

$ pyright playground/ards.py
0 errors, 0 warnings, 0 informations

This is very basic, but I've ended up with cast(Sequence, ds) in several places.

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📚 Documentation preview 📚: https://google-grain--1190.org.readthedocs.build/