BUG: DataFrameGroupBy agg with multi-column object block
This is analogous to what we did recently-ish with DataFrame._reduce.  When we see an object block, split it and operate column-wise.  Side-bonus: this makes the BM behavior match the AM behavior.
is this a user facing bug ? can you add a whatsnew whatsnew added + green gentle ping; will need to rebase #41147 after this