Add topk detection in build_spatiotemporal_context #498

Part of #497.

build_spatiotemporal_context (asap-query-engine/src/engines/simple_engine/sql.rs) needs to detect topk/bottomk-shaped queries so they can be folded into the SpatioTemporal query type instead of relying on nested-aggregation handling.

Blocks the removal of Spatial/TemporalGeneric/TemporalQuantile/OneTemporalOneSpatial detection (separate child issue) — that removal needs this in place first.