Separate walk time from non-transit time

[leonardehrenfried]

Summary

@amy-corson-ibigroup has reported that the GTFS API falsely returns the non-transit duration as the walk one.

This is fixed by this PR.

This is just a minimal PR to fix the actual issue, but I want to create a follow-up where I rework this system and make a more general API for show all sorts of durations for walking, biking and driving.

Unit tests

✔️

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (14aca8a) 67.54% compared to head (7fc2e87) 67.54%.
Report is 1 commits behind head on dev-2.x.

Additional details and impacted files

@@            Coverage Diff             @@
##             dev-2.x    #5648   +/-   ##
==========================================
  Coverage      67.54%   67.54%           
- Complexity     16293    16297    +4     
==========================================
  Files           1888     1888           
  Lines          71609    71617    +8     
  Branches        7405     7406    +1     
==========================================
+ Hits           48365    48373    +8     
+ Misses         20741    20740    -1     
- Partials        2503     2504    +1     

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