feat(gc): add statement-level trigger support for layer deletions

This MR updates the gc_track_deleted_layers database function to support both statement-level and row-level trigger calls. The rationale is explained in #732 (comment 1042436138), and the actual change was devised from #732 (comment 1042923125).

There will be another MR to actually change the gc_track_deleted_layers_trigger trigger (the caller for this function) from a row-level to a statement-level execution. The idea is to split the release and deployment of this change in a safer/iterative way. We can then optionally follow up with a third MR to remove support for row-level execution on the gc_track_deleted_layers function once the previous two are successfully deployed.

Related to #732 (closed).