Support adding arbitrary data to models

[ianthetechie]

A member of the Slack channel noted that their router included additional information with route steps that they wanted to access in their application. In this case it was traffic congestion levels, but this is something we should do anyways even if we end up adding that to the core model.

Ideally this will be arbitrary key/value pairs that support anything JSON, but we may hit some implementation snags with that in Rust / UniFFI. We could work around this by storing a JSON string internally and exposing computed properties in Swift and Kotlin.

[ianthetechie]

After further discussion it seems this probably applies to other models too like waypoints.