Interpret pbs_birth_cohort flag during PSC sync. Closes #3510.

This new flag is not currently ever set by `Psc::SyncLoader`. This means that eligibility events will always be sync'd using the PBS provider-recruited PSC segment.

In the future (#3512) we will need to determine how to set this flag appropriately in `Psc::SyncLoader`. For now we are not supporting birth cohort syncing from Field.