editoast: migrate endpoint to create a rolling_stock livery

[clarani]

closes #3498

⚠️ this new endpoint is not tested because TestRequest does not support multipart for the moment. Here is a ticket #3931 to add tests on this specific endpoint (in the future).

To test this endpoint, you can use postman with this configuration (select form-data for the body, use the images in editoast/src/tests if you want). You can add several images for a single request.

[clarani]

Ready for review

[codecov]

Codecov Report

Merging #3892 (394be8b) into dev (46afe1c) will decrease coverage by 0.34%.
The diff coverage is 27.95%.

❗ Current head 394be8b differs from pull request most recent head 3404a48. Consider uploading reports for the commit 3404a48 to get more accurate results

@@             Coverage Diff              @@
##                dev    #3892      +/-   ##
============================================
- Coverage     69.83%   69.50%   -0.34%     
  Complexity     2026     2026              
============================================
  Files           441      439       -2     
  Lines         22432    22332     -100     
  Branches       1672     1672              
============================================
- Hits          15665    15521     -144     
- Misses         5962     6006      +44     
  Partials        805      805              

Flag	Coverage Δ
editoast	72.84% <5.97%> (-1.95%)	⬇️
front	59.33% <84.61%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
editoast/src/models/mod.rs	37.03% <ø> (ø)
editoast/src/models/rolling_stock/mod.rs	96.87% <ø> (ø)
...t/src/models/rolling_stock/rolling_stock_livery.rs	33.33% <ø> (ø)
editoast/src/tables.rs	0.00% <ø> (ø)
editoast/src/views/rolling_stocks.rs	35.35% <5.97%> (-61.62%)	⬇️
front/src/common/api/osrdEditoastApi.ts	91.30% <84.61%> (-0.33%)	⬇️

... and 10 files with indirect coverage changes

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[younesschrifi]

LGTM !! Nice PR :)

[flomonster]

Thanks for this PR! This looks good to me. I've suggested changes to slightly improve the code.