Pathfinder: Update TRAPI input and output format

[tokebe]

The Pathfinder Working Group has moved toward finalizing a schema for pathfinder queries and their responses. It's expected that the TRAPI Working Group will start up and further update TRAPI to include this schema. In the meantime, Abrar will be making an implementation guide.

This issue will be updated as these references become available, and will track work to update BTE's Pathfinder implementation to conform to the new schema.