[backend] improve file indexing metrics performance (#7530)

[SouadHadjiat]

Proposed changes

• use aggregation to count files by mime type

Related issues

• File indexing page is too slow to be production grade ready #7530

performance issue not reproduced locally => need to test queries directly on DB (with platform team)

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality
• I wrote test cases for the relevant uses case (coverage and e2e)
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality

[codecov]

Codecov Report

Attention: Patch coverage is 30.76923% with 18 lines in your changes missing coverage. Please review.

Project coverage is 67.55%. Comparing base (335977c) to head (8e6b3bc).
Report is 1 commits behind head on master.

Files	Patch %	Lines
...l/src/modules/internal/document/document-domain.ts	20.00%	12 Missing ⚠️
...pencti-platform/opencti-graphql/src/domain/file.js	14.28%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7556      +/-   ##
==========================================
- Coverage   67.55%   67.55%   -0.01%     
==========================================
  Files         567      567              
  Lines       69903    69917      +14     
  Branches     5937     5938       +1     
==========================================
+ Hits        47225    47230       +5     
- Misses      22678    22687       +9     

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[JeremyCloarec]

The files count is always set to 1 on my file indexing page