Feat/realtime tripupdates builder eta estimator subissue 16

[jloria2703]

Sub-issue #16 — Build TripUpdates with ETA Module

Work focused on implementing the ETA-aware TripUpdates builder using a simulated Bytewax-style flow.
The goal was to construct TripUpdate entities enriched with Estimated Time of Arrival (ETA) predictions, while still working with synthetic data instead of real database-backed journeys.

To validate the dataflow before having real Journey / StopTimes models wired in, several helper files were created:

1. fake_stop_times.py: generates synthetic stops with deterministic arrival times.
2. stop_times.py: provides the helper function estimate_stop_times() that returns predefined ETA values.
3. trip_update_fixture.json: acts as a synthetic input fixture to feed build_trip_updates_bytewax() when there are no real Journey or StopTimes objects available.

Execution

The module can be executed interactively through the Django shell using the following commands:

python manage.py shell

>>> from gtfs.utils import realtime
>>> realtime.build_trip_updates_bytewax()