core: allow LongString in the x-rpc-path header

[Khoyo]

The RabbitMQ management client doesn't allow us to set the headers as bytes, so we should be more lenient as to the received data type.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 42.50%. Comparing base (cd3935d) to head (3347028).
Report is 6 commits behind head on dev.

Files with missing lines	Patch %	Lines
...re/src/main/java/fr/sncf/osrd/cli/WorkerCommand.kt	0.00%	3 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff              @@
##                dev    #9596      +/-   ##
============================================
- Coverage     42.51%   42.50%   -0.01%     
  Complexity     2271     2271              
============================================
  Files          1311     1311              
  Lines        105408   105410       +2     
  Branches       3296     3297       +1     
============================================
- Hits          44813    44805       -8     
- Misses        58649    58659      +10     
  Partials       1946     1946              

Flag	Coverage Δ
core	75.05% <0.00%> (-0.02%)	⬇️
editoast	73.67% <ø> (-0.03%)	⬇️
front	18.79% <ø> (ø)
gateway	2.19% <ø> (ø)
osrdyne	3.28% <ø> (+<0.01%)	⬆️
railjson_generator	87.49% <ø> (ø)
tests	86.74% <ø> (ø)

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