least (copy), greatest (copy) and dateadd mock

[osipovartem]

• Copied least and greatest UDF functions from this PR (already merged, but not released). After datafusion release we should remove this local copy
• Added mock for dateadd function to support sql syntax
• Changed logic for datafusion -> snowflake timestamps convertor for a while (still receive the error 252005: Failed to convert: field min_last_success: TIMESTAMP_NTZ::min_last_success, Error: invalid literal for int() with base 10: 'min_last_success')

[osipovartem]

/ptal @slyons @DanCodedThis

[DanCodedThis]

For dateadd we can either use utf8 or it's variants or extend the sql parser to actualy work with underlying types, but the addition operator should work out of the box, more info in issue #68 (keep in mind there are two branhces now)

[DanCodedThis]

Also when using nanoseconds, maybe I'm using to small of a value but I can't seem to see the change, or is it a part of that "thing" where timestamps don't work with them yet?

[DanCodedThis]

And us (microsenconds) also

[rampage644]

Closes #75

[DanCodedThis]

i64 max is 9_223_372_036_854_775_807, and since we are adding nanoseconds even with years, if somebody decides to add 30 years to any timestamp/date/time we will have an overflow (in the value that is being added, not the timestamp), we are flying dangerously close to the sun for my liking, or are we?

I initially decided not to use MonthDayNano since days and month are i32, and even though we distinguish years, month, weeks, etc., anybody by mistake can write any value that may be bigger than i32 max, accidentally (since we always receive i64, ex.: was writing nanoseconds switched the form ns to yy) or is it just an edge case?

Plus the other problem is leap years, and "leap months", which should be handled internally by timestamps (didn't check yet, but it should be common sense), but because we are adding nanoseconds, our months are always 30 days and our years are always 365 days.

I'm going to look into this, and after that the PR should be ready

[DanCodedThis]

Dateadd completed

[DanCodedThis]

Dateadd completed

[DanCodedThis]

/ptal @osipovartem