Report steered pathway ID via LoadEventInfo

Add a field `steeredPathwayId` to `LoadEventInfo`, which is populated from the `ParsingLoadable` and `Chunk` for media playlists and segments.

PiperOrigin-RevId: 942243247
(cherry picked from commit de4022b0ba834e6964cb3f2a7fbeff8d3c2b5a07)