TS2: Add core signal projection endpoint

[flomonster]

Description

Add an endpoint POST /v2/project_signals` which does quite the same thing as the v1.

AC

• [ ] The endpoint takes a simplified train path (only track or routes TBD)
• [ ] The endpoint response
  • Inputs: no flatten, starting datetime of trains and timedelta in seconds (f64)
  • Outputs: the smallest datetime and timedelta everywhere in seconds (f64)