Improve aggregate functions for temporals

[mcrumiller]

Description

This is an overarching issue to keep track of a series of sub-issues related to temporals. Temporal objects should have well-defined mean/median/std/var/quantile definitions, but the majority of these are either missing from the API or are inconsistently implemented with respect to series, expressions, and literals.

The following matrix shows the current progress on these sub-issues. Most of these have already have implementations in the queue so please consult before diving in with an implementation.

Dtype	Mean/Median	Std/Var	Quantile
Date	Issue #13598 | PR #16795 (merged)	-	-
Datetime	Issue <none> | PR #13417 (merged)	-	-
Duration	Issue #14375 | PR #14376 (merged)	-	-
Time	Issue #14470 | PR #14471 (merged)	-	-