builtins: implement aggregates sum, product and sort

[DFrenkel]

What is the underlying problem you're trying to solve?

Need implementation of product, sum and sort to be used in policies

Expected Outcome

• product, sort and sum can be used in policies
• all aggregates tests pass: https://github.com/open-policy-agent/opa/tree/main/v1/test/cases/testdata/v1/aggregates (note that this will require also implementing internal.member_3)