Buoyed by more than $7 million in federal and state aid, a prominent Farmington Valley developer recently kicked off work to build a 90-unit affordable housing development in Farmington that will include 20 supportive apartments for people with developmental disabilities. A fleet of heavy construction trucks has been doing site work since October at 1600 New Britain Ave., a 9-acre tract that’s just north of Route 6 and about a mile and a half fr…