REF: move ArrowStringArray.__setitem__ and related methods to ArrowExtensionArray
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Moved/refactored `ArrowStringArray.__setitem__` and related methods to `ArrowExtensionArray`. These methods should be generic enough to support any pyarrow type. I left `_maybe_convert_setitem_value` to be implemented by the subclasses as I figured there may need to be type specific logic there.
thanks @lukemanley 