Encapsulate time range constraints for predicate pushdown (#1215)

MetricFlow currently allows for a limited scope form of filter
predicate pushdown that is particular to time range constraints where
the querying user has provided an explicit time window for us to
query against. The application of the pushdown operation is managed
by threading the time range constraint through the entire dataflow
plan builder and applying the filter operation as appropriate.

This is exactly what we need to do for robust predicate pushdown
evaluation for our expanded set of pushdown operations. Rather than
wire a whole new set of parameters through, we simply encapsulate
the time range constraints inside of a new object that is more
readily extensible to other predicate pushdown handling.

This specific change is as mechanical as possible in order to minimize
confusion. Places that stood out for improvement via the encapsulating
object have been marked for later updates, which will follow shortly.