### What changes were proposed in this pull request?
Add `aggregate` package under `sql/catalyst/src/main/java/org/apache/spark/sql/connector/expressions` and move all the aggregates (e.g. `Count`, `Max`, `Min`, etc.) there.
### Why are the changes needed?
Right now these aggregates are under `sql/catalyst/src/main/java/org/apache/spark/sql/connector/expressions`. It looks OK now, but we plan to add a new `filter` package under `expressions` for all the DSV2 filters. It will look strange that filters have their own package, but aggregates don't.
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
Existing tests
Closes#33815 from huaxingao/agg_package.
Authored-by: Huaxin Gao <huaxin_gao@apple.com>
Signed-off-by: Liang-Chi Hsieh <viirya@gmail.com>
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