cleanup FlightDataPath array access/slicing (#69241)
Working with `FlightDataPath` and `FlightSegmentPath` is tricky because
they're typed as `any[]`. Array access and slicing is sprinkled
throughout the router code which makes it difficult to
add/remove/re-order properties. It also results in more untyped code,
because anything that touches `FlightDataPath` becomes `any`.
To improve the DX around working with this data structure, I'm starting
by moving all of the array slicing/property access into a single
location. And `getFlightDataPartsFromPath` provides types for the
different properties it returns so things like `seedData` aren't `any`.
There's currently two similarly named but different paths that get
returned by this function to support forked handling (ppr vs non-PPR):
`pathToSegment` and `segmentPath`. Open to alternative names here, but
my goal in an upcoming PR is to just have a single segment path and
unify the handling. To get there, I first need to investigate all the
`// TODO-APP remove ''` comments because we're inconsistent with how the
segment path is constructed & consumed.
