[FEATURE] PPL `earliest`/`latest` aggregation function support

[xinyual]

Is your feature request related to a problem?
We need to support earliest/latest as aggregate function, it calcite the earliest/latest time for a time related column. Also, it should be supported in eventstat as well

[dai-chen]

@LantaoJin @xinyual I'm doing gap analysis for this and have questions below. As I understand, PR #3640 adds earliest and latest function as scalar conditional function.

1. This issue is not resolved right? Shall we reopen it?
2. Could you share the SPL doc that earliest and latest as conditional function?