core: stdcm: reorganize step data to make them dynamic

[eckter]

Fix #8974

Follow-up of #10989 (which started the wrong way).

May be reviewed commit by commit, but there's been a few changed from earlier commits that have been changed again in later ones. But they're mostly self-contained changes and each commit normally compiles and runs properly (though I've only run unit tests).

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The main goal of this refactor was to remove the use of step data as a static thing, so that they can be changed dynamically when implementing overtakes. The main challenge was checking for graph.steps to test for, e.g., stop duration. A few data redundancy have been removed as well (like stop offsets in InfraExplorerWithEnvelope), but they weren't too tangled up.

The main change is the Introduction of a class that keeps track of the steps on the path, StepTracker. It includes both stops and non-stop waypoints, while keeping track of what's in the lookahead section. Eventually it may include stops that aren't part of the input.

This solves two main issues: we don't need to check for the input step list anymore, we can instead rely on StepTracker's step list, which would include extra stops. We also don't need to manually keep track of waypoint indexes in edges/nodes (which again wouldn't match with a more dynamic stop list).

There's some more minor refactoring along the way.

[codecov-commenter]

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All modified and coverable lines are covered by tests ✅

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@@            Coverage Diff             @@
##              dev   #11109      +/-   ##
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+ Coverage   80.86%   80.88%   +0.01%     
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  Files        1118     1119       +1     
  Lines      112736   112834      +98     
  Branches      759      759              
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+ Hits        91164    91262      +98     
  Misses      21517    21517              
  Partials       55       55              

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front	89.90% <ø> (+0.01%)	⬆️
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railjson_generator	87.58% <ø> (ø)
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[eckter]

Having to always keep the current isntance in a consistent state even when it's meant to be discarded was a lot more painful here. We need to update the step tracker to properly handle the following edges.

[eckter]

TODO (?)

We still use raw input steps in BlockAvailability

[eckter]

Suggested change

// TODO: maybe integrate this logic to StepTracker?