REF: Make function for dtype casting in pre-processing of groupby

[phofl]

Is your feature request related to a problem?

[this should provide a description of what the problem is, e.g. "I wish I could use pandas to do [...]"]

Describe the solution you'd like

As mentioned in #37145 the pre processing part in

pandas/pandas/core/groupby/groupby.py

Line 2174 in 6abe510

elif is_datetime64_dtype(vals.dtype):

is very generic and could be moved to a function

[jbrockmendel]

@phofl i think #51003 would close this, can you confirm