[ArrayManager] GroupBy cython aggregations (no fallback)
xref https://github.com/pandas-dev/pandas/issues/39146/

This implements one aspect of groupby: basic cython-based aggregations (so not yet general apply, python fallback, or other methods, etc, only the basic aggregations that take the `_cython_agg_general` path).

Similarly to how we currently have a `_cython_agg_blocks`, this PR adds an equivalent `_cython_agg_arrays` which calls the cython operation on each column instead of each block.