main: content: allow reconcile pushdown_strategy to be set via DagProperties
As part of T411803, we want to test whether changing the pushdown_strategy on reconcile event ingestion helps us ingest 40M+ remaining reconcile events.
The idea here is that, on regular runs, this will continue to be set to default of set_of_page_ids, while on really big reconcile events it will be manually set to earliest_revision_dt.
Bug: T411803