stdcm: implement backtracking

[eckter]

This change lets us modify the previous edges, which is used to fix speed discontinuities.
It will also let us use engineering margins in a future PR.

Short summary of changes:

• The edges keep track of the previous edges / nodes and we now reassemble the path ourselves at the end
• When there is a speed discontinuity, we create a "new" previous edge with the appropriate slowdown
• Some code was moved around, inner classes of STDCMGraph were extracted, code sections have been added

[codecov]

Codecov Report

Merging #2201 (0c1666e) into dev (e097895) will increase coverage by 0.21%.
The diff coverage is 93.79%.

@@             Coverage Diff              @@
##                dev    #2201      +/-   ##
============================================
+ Coverage     78.87%   79.09%   +0.21%     
- Complexity     1669     1696      +27     
============================================
  Files           274      277       +3     
  Lines          7575     7659      +84     
  Branches        945      959      +14     
============================================
+ Hits           5975     6058      +83     
+ Misses         1252     1251       -1     
- Partials        348      350       +2     

Flag	Coverage Δ
core	82.20% <93.79%> (+0.23%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ain/java/fr/sncf/osrd/api/stdcm/STDCMEndpoint.java	84.37% <ø> (ø)
...n/java/fr/sncf/osrd/api/stdcm/graph/STDCMEdge.java	50.00% <50.00%> (ø)
...fr/sncf/osrd/api/stdcm/graph/STDCMPathfinding.java	98.03% <94.28%> (ø)
.../java/fr/sncf/osrd/api/stdcm/graph/STDCMGraph.java	98.15% <97.93%> (ø)
.../sncf/osrd/api/stdcm/BacktrackingEnvelopeAttr.java	100.00% <100.00%> (ø)
...n/java/fr/sncf/osrd/api/stdcm/graph/STDCMNode.java	100.00% <100.00%> (ø)
...d/infra_state/implementation/TrainPathBuilder.java	83.33% <0.00%> (+1.38%)	⬆️

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