Cosmos: DateTimeOffset with non-zero offsets are not supported by functions

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The Cosmos provider allows inserting DateTimeOffsets with arbitrary offsets, but the Cosmos functions don't appear to support them:

SELECT DateTimePart('hh', '2020-01-01 12:30:15') -- 12
SELECT DateTimePart('hh', '2020-01-01 12:30:15Z') -- 12
SELECT DateTimePart('hh', '2020-01-01 12:30:15+02:00') -- undefined

Unfortunately, even for zero offsets, we generate e.g. 0010-05-03T12:45:38+00:00, which isn't supported; we need to generate 0010-05-03T12:45:38Z instead. We could consider not allowing DateTimeOffsets with non-zero offsets.