Add group by on composite type support.

We currently default to hash aggregates in the master planner. If the column doesn't have a hash operator (which composite types don't), then we get error out.

Possible solutions;

i. In BuildAggregatePlan(), allow grouped aggregates also to use AGG_SORTED. (Most likely there should some reason to pick AGG_HASHED by default, but I don't know what it is)

ii. Add hash support for composite types in PostgreSQL. I remember we talked about this probably in a different context, and Jason (or somebody else) coded a prototype. Don't know what happened after that.