stdcm linear allowance

[eckter]

This PR adds linear allowance in STDCM requests (back-end only).

We don't directly change the envelopes as we create them, because we need to keep track of the original envelope to properly compute the allowance and handle route transitions.
Instead, we keep track of the speed factor as a new field. We use it whenever we compute the time it takes to go from point A to B.

[codecov]

Codecov Report

Merging #2645 (07e80ba) into dev (9d2ec15) will increase coverage by 0.18%.
The diff coverage is 96.61%.

@@             Coverage Diff              @@
##                dev    #2645      +/-   ##
============================================
+ Coverage     38.05%   38.24%   +0.18%     
- Complexity     1678     1694      +16     
============================================
  Files           576      578       +2     
  Lines         17747    17803      +56     
  Branches       2446     2451       +5     
============================================
+ Hits           6754     6809      +55     
- Misses        10607    10608       +1     
  Partials        386      386              

Flag	Coverage Δ
core	83.92% <96.61%> (+0.13%)	⬆️

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

Impacted Files	Coverage Δ
.../java/fr/sncf/osrd/api/stdcm/graph/STDCMUtils.java	92.85% <ø> (-0.90%)	⬇️
...ljson/parser/RJSStandaloneTrainScheduleParser.java	68.05% <ø> (ø)
...e/src/main/java/fr/sncf/osrd/utils/SwingUtils.java	0.00% <ø> (ø)
...ain/java/fr/sncf/osrd/api/stdcm/STDCMEndpoint.java	77.77% <33.33%> (-1.94%)	⬇️
...fr/sncf/osrd/api/stdcm/graph/AllowanceManager.java	91.89% <88.88%> (-0.42%)	⬇️
.../java/fr/sncf/osrd/api/stdcm/graph/STDCMGraph.java	96.55% <90.00%> (-3.45%)	⬇️
...fr/sncf/osrd/api/stdcm/graph/STDCMSimulations.java	97.50% <97.50%> (ø)
...sncf/osrd/api/stdcm/graph/STDCMPostProcessing.java	98.68% <98.68%> (ø)
...main/java/fr/sncf/osrd/api/stdcm/STDCMRequest.java	100.00% <100.00%> (ø)
...sncf/osrd/api/stdcm/graph/BacktrackingManager.java	96.77% <100.00%> (-0.67%)	⬇️
... and 8 more

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