I think this would be an ambitious, but doable first issue.

There's a handful of things that need to happen for this:

• We need a public interface from pkg/util/json to incrementally construct a
  JSON object/array. In the case of objects, since their keys need to be
  sorted, we would want to do something like push the new keys into a priority
  queue (the Go stdlib has a heap implementation). This should be tested
  independently from SQL-land. I think this will probably be useful outside of
  these aggregates so it should be a somewhat general interface.
• Add the aggregates in aggregate_builtins.go that use these new constructors.
  These will need be careful to monitor their memory usage, see the
  implementation of ARRAY_AGG for reference.
• Test the aggregates in SQL-land. Care should be taken to compare the
  semantics of what is implemented with what Postgres does (handling of NULL,
  duplicate keys, etc.).

This needs to happen for the next major release so I'll get to it if nobody is
interested in picking it up.