Support aggregates with stype = internal

Now, aggregation functions that support parallel execution in Citus are configured through a hard-coded whitelist in `multi_logical_optimizer.h`.

We need to use the aggregate function in `pg_roaringbitmap`

https://github.com/ChenHuajun/pg_roaringbitmap
-`rb_and_agg`
-`rb_or_agg`

Could this simple whitelist of aggregation functions satisfying the associativity be made configurable?

Read multi_logical_optimizer.h and trace the hard-coded aggregate whitelist. Check how rb_and_agg and rb_or_agg from pg_roaringbitmap would be represented; done means the associative aggregate set is configurable rather than fixed in that whitelist.

Written by the indexing model from the issue text.