[SPARK-40744][PS] Make `_reduce_for_stat_function` in `groupby` accept `min_count`
[SPARK-40744][PS] Make `_reduce_for_stat_function` in `groupby` accept `min_count`

### What changes were proposed in this pull request?
Make `_reduce_for_stat_function` in `groupby` accept `min_count`

### Why are the changes needed?
to simplify the implementations

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
existing UTs

Closes #38201 from zhengruifeng/ps_groupby_mc.

Authored-by: Ruifeng Zheng <21ef94aec54fae1a36abbd011347596ae66d9959@apache.org>
Signed-off-by: Hyukjin Kwon <2680965ad5aae29275c8fb10150cb10f5554e78b@apache.org>

mahak/spark,hvanhovell/spark,dongjoon-hyun/spark,HyukjinKwon/spark,mahak/spark,wangyum/spark,dongjoon-hyun/spark,shaneknapp/spark,apache/spark,shaneknapp/spark,HyukjinKwon/spark,shaneknapp/spark,zzcclp/spark,ueshin/apache-spark,srowen/spark,HyukjinKwon/spark,zzcclp/spark,HyukjinKwon/spark,mahak/spark,apache/spark,hvanhovell/spark,vinodkc/spark,ueshin/apache-spark,hvanhovell/spark,dongjoon-hyun/spark,hvanhovell/spark,dongjoon-hyun/spark,srowen/spark,dongjoon-hyun/spark,hvanhovell/spark,zzcclp/spark,ueshin/apache-spark,srowen/spark,vinodkc/spark,wangyum/spark,vinodkc/spark,apache/spark,wangyum/spark,dongjoon-hyun/spark,srowen/spark,wangyum/spark,vinodkc/spark,srowen/spark,HyukjinKwon/spark,hvanhovell/spark,zzcclp/spark,vinodkc/spark,ueshin/apache-spark,wangyum/spark,HyukjinKwon/spark,shaneknapp/spark,zzcclp/spark,shaneknapp/spark,mahak/spark,zzcclp/spark,mahak/spark,zzcclp/spark,srowen/spark,wangyum/spark,dongjoon-hyun/spark,HyukjinKwon/spark,ueshin/apache-spark,srowen/spark,shaneknapp/spark,mahak/spark,apache/spark,vinodkc/spark,hvanhovell/spark,ueshin/apache-spark,apache/spark,shaneknapp/spark,mahak/spark,apache/spark,vinodkc/spark,wangyum/spark,apache/spark,ueshin/apache-spark