[backend] Improv performance on challenges

[RomuDeuxfois]

#898

[codecov]

Codecov Report

Attention: Patch coverage is 72.88136% with 16 lines in your changes missing coverage. Please review.

Project coverage is 26.36%. Comparing base (9defb91) to head (4e6eab1).

Files	Patch %	Lines
...o/openbas/rest/challenge/ExerciseChallengeApi.java	10.00%	9 Missing ⚠️
...ava/io/openbas/rest/challenge/ChallengeHelper.java	66.66%	2 Missing and 1 partial ⚠️
...openbas/rest/challenge/output/ChallengeOutput.java	89.65%	2 Missing and 1 partial ⚠️
...n/java/io/openbas/rest/challenge/ChallengeApi.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1066      +/-   ##
============================================
+ Coverage     25.87%   26.36%   +0.49%     
- Complexity     1007     1024      +17     
============================================
  Files           451      455       +4     
  Lines         10568    10621      +53     
  Branches        513      514       +1     
============================================
+ Hits           2734     2800      +66     
+ Misses         7771     7756      -15     
- Partials         63       65       +2     

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[Christian-DONGMO]

Everything sound good