DOC/TST: provide documentation & testing on groupby filtering & aggregation ops for EA

[jreback]

#27071 allows for the preservation dtype for filtering type of ops in groupby (first, last, min, max). This should work for all EA (though was fixing Categorical).

We should

• test all EA more generally for groupby filtering type ops & potentially other aggregations
• document what works / what doesn't work.

[kidrahahjo]

@jreback will it be okay to close this issue? Or some work is required to be done here?

[jreback]

yep i think this is good