feat(rust, python): Return datetime for mean/median of Date colum

[mcrumiller]

Old PR was pretty stale, so here is a new one.

Resolves #13598. Part of overall #13599.

This treats dates and datetimes as analagous to integers/floats: the mean of ints is a float, and now the mean of dates is a datetime:

mean(1, 2) = 1.5
mean(2023-01-01, 2023-01-02) = 2023-01-01 12:00.

This is the last temporal mean/median PR, which means a lot of custom logic for the different temporal types has been collapsed/simplified. If this is accepted, we can deprecate dt.mean and dt.median, as mean/median for all temporals have now been implemented, they are now functionally equivalent to series.mean and series.median for all temporal dtypes. I'll do a followup on std/var/quantile.

Note that this breaks streaming date mean/median, and so for the time being I have set it to return a null series. I don't want to hack in a solution since a new streaming engine is being developed, so in the meantime I'm not completely sure what to do.

[codspeed-hq]

CodSpeed Performance Report

Merging #16795 will degrade performances by 16.52%

_{Comparing mcrumiller:date-mean (1ce4c0b) with main (fd4c71e)}

Summary

⚡ 1 improvements
❌ 1 regressions
✅ 35 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	mcrumiller:date-mean	Change
❌	test_groupby_h2oai_q5	1.9 ms	2.3 ms	-16.52%
⚡	test_tpch_q14	2.5 ms	2.1 ms	+17.45%