Add return_not_null_data_as_optional parameter to read_table

[krasnovdm]

Problem

read_table always returns NOT NULL columns wrapped in Optional, and there is no way to opt out in the Python SDK.

The gRPC ReadTableRequest already has a return_not_null_data_as_optional field, and the C++ SDK exposes it via TReadTableSettings().ReturnNotNullAsOptional(false). The Python SDK simply never set this field, so users reading a table with NOT NULL columns always get Optional values back.

Change

Add a return_not_null_data_as_optional parameter to read_table (sync ydb/table.py and async ydb/aio/table.py) and plumb it through read_table_request_factory. It accepts a bool (mapped to FeatureFlag.ENABLED/DISABLED) or a raw FeatureFlag.Status.

Default behavior is unchanged: when the parameter is not provided, the field is left unset and NOT NULL columns are still returned as Optional (backward compatible).

# opt out of the Optional wrapping for NOT NULL columns:
session.read_table(path, return_not_null_data_as_optional=False)