editoast: migrate endpoint GET rolling_stock/{id}/livery/{id}

[clarani]

close #3300

• migrate endpoint GET rolling_stock/{id}/livery/{id} to rust
• remove delete on cascade on rolling_stock_compound_image (otherwise, when we delete the compound image, it automatically deletes the livery too, which is not the expected behaviour)
• add tests & documentation

Later, we should change the database structure to use the delete on cascade (which implies removing the compound_image_id of the livery and add the livery_id in the compound_image = reverse the relationship)

[codecov]

Codecov Report

Merging #3519 (15560a3) into dev (6ad9b2c) will increase coverage by 0.07%.
The diff coverage is 84.46%.

@@             Coverage Diff              @@
##                dev    #3519      +/-   ##
============================================
+ Coverage     68.31%   68.38%   +0.07%     
  Complexity     1896     1896              
============================================
  Files           413      415       +2     
  Lines         20606    20702      +96     
  Branches       1519     1519              
============================================
+ Hits          14076    14158      +82     
- Misses         5829     5843      +14     
  Partials        701      701              

Flag	Coverage Δ
editoast	75.03% <83.14%> (+0.21%)	⬆️
front	56.52% <92.85%> (+0.05%)	⬆️

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

Impacted Files	Coverage Δ
editoast/src/documents.rs	80.55% <ø> (ø)
editoast/src/schema/mod.rs	94.44% <ø> (ø)
editoast/src/schema/rolling_stock.rs	88.46% <ø> (ø)
editoast/src/views/light_rolling_stocks.rs	100.00% <ø> (ø)
editoast/src/schema/rolling_stock_image.rs	72.50% <72.50%> (ø)
editoast/src/schema/rolling_stock_livery.rs	87.50% <87.50%> (ø)
front/src/common/api/osrdEditoastApi.ts	92.58% <92.85%> (+0.09%)	⬆️
editoast/src/views/rolling_stocks.rs	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[flomonster]

Good for me but this code will be highly refactored.
We should store liveries images in the document table.
We will use the new model system to interact with the DB.

[Castavo]

Nice PR !
I've learnt about Selectable today 😁