core: convert envelope files to kotlin

[eckter]

Part of #10991, just for an easier way to review and comment this

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.83%. Comparing base (1b37d58) to head (4833837).
Report is 10 commits behind head on dev.

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Additional details and impacted files

@@           Coverage Diff           @@
##              dev   #11120   +/-   ##
=======================================
  Coverage   80.83%   80.83%           
=======================================
  Files        1115     1115           
  Lines      112495   112509   +14     
  Branches      759      759           
=======================================
+ Hits        90934    90946   +12     
- Misses      21506    21508    +2     
  Partials       55       55           

Flag	Coverage Δ
editoast	72.46% <ø> (-0.01%)	⬇️
front	89.89% <ø> (+<0.01%)	⬆️
gateway	2.49% <ø> (ø)
osrdyne	2.53% <ø> (ø)
railjson_generator	87.58% <ø> (ø)
tests	88.81% <ø> (ø)

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[Erashin]

Yeah ok ngl I only took a couple details into account. The rest kinda fuck the java part up, and ngl, this ain't my code, this is just a kotlin reformat. We discussed it with @eckter we agree it's fine as is.
Taken into account in the origin pr.