Fix dateadd function

[rampage644]

This is a quick fix for dateadd UDF - it just makes it work such that its physical execution works for dbt snowplow
workload.

• Use interval instead of duration type for hour/minute/second
• Handle empty array case

Ideally, it should be reimplemented differently:

• Non-deprecated methods should be used
• It should be designed to handle ColumnarValue::Array as a first class citizen (whereas scalar is an exception, not otherwise)
• (Debatable) Perhaps even 2nd argument should be treated as a columnar value, not scalar

Good example is in datafusion-examples directory.

Fixes - somewhat incorrect - #223

[DanCodedThis]

• Arrays are first citizens now, and array length bug should be fixed now
• For now I implemented a simple fix for returning Scalar vs Array, from Artem's suggestion: https://github.com/Embucket/datafusion/blob/icehut/datafusion/functions/src/regex/regexpsubstr.rs#L111
• DF says for perf reasons it's better to have different impl's for them, so later we can explore that to speed it up (if it's slow)

[Vedin]

It's fine for now and I believe work pretty fast. I still don't like to create additional array for scalar values. We should consider using something similar to evaluate_array_scalar in the future.

[rampage644]

@DanCodedThis Feel free to take over this (i.e. decision to merge is on you), feel free to remove all code I've written altogether.