[BUG] Resolve minor scalar timestamp bugs

[sarahyurick]

https://github.com/dask-contrib/dask-sql/pull/1025 enhanced our logic in dealing with scalar timestamps. However, there are still a couple of bugs as marked in test_scalar_timestamps:

• [ ] SELECT TIMESTAMPDIFF(DAY, to_timestamp(d1), to_timestamp(d2)) AS dt FROM df errors on the GPU side
• [ ] SELECT CAST({scalar1} AS TIMESTAMP) + INTERVAL '5 days' AS dt has issues relating to performing a seconds/nanoseconds conversion
• [ ] SELECT EXTRACT(DAY FROM CAST({scalar1} AS TIMESTAMP)) AS day also has issues performing a seconds/nanoseconds conversion