Enable spilling support for partial aggregation

[zhztheplayer]

Description

For more earlier discussion, refer to #7511

Partial aggregation is expected to reduce data size transmitted from mapper to reducer. So far Velox's partial aggregation flushes intermediate data out once memory usage reaches limit, so more or less the data reduction ratio will be subject to the frequency of flushing.

This issue would introduce the possibility of brining spilling support to partial aggregation, to make it be able to maintain the lowest data reduction ratio despites the memory resource limit. This could be useful when:

1. When reducer number is far less than mapper number
2. When network between mappers and reducers is bad
3. When shuffle/exchange cost between mappers and reducers is high

In the fix, we'd introduce an option which is by default false to enable spilling support for partial aggregation. When the option is true, disable flushing at the same time.

[zhztheplayer]

Closing as not planned. See background here.