Support snapshot-rw isolation mode in query client in Python SDK

[zverevgeny]

API:
https://github.com/ydb-platform/ydb/blob/8ed59d59c5dec4e4456fef8bea4f4220fbc764f4/ydb/public/api/protos/ydb_query.proto#L67
https://github.com/ydb-platform/ydb/blob/8ed59d59c5dec4e4456fef8bea4f4220fbc764f4/ydb/public/api/protos/ydb_table.proto#L1060

C++ SDK already supports this mode