[frontend] Add 'Unknown' value in Main Observable Type filter values (#10145)

[Archidoit]

Proposed changes

Add 'Unknown' value in 'Main observable type' filter values (Indicators list).

Related issues

fixes #10145

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 33.61%. Comparing base (2ff2889) to head (2cdfbab).

Files with missing lines	Patch %	Lines
...ncti-front/src/utils/filters/useSearchEntities.tsx	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #15595      +/-   ##
==========================================
- Coverage   33.61%   33.61%   -0.01%     
==========================================
  Files        3189     3189              
  Lines      216746   216748       +2     
  Branches    39951    39951              
==========================================
  Hits        72856    72856              
- Misses     143890   143892       +2     

Flag	Coverage Δ
opencti-client-python	45.60% <ø> (ø)
opencti-front	2.94% <0.00%> (-0.01%)	⬇️
opencti-graphql	70.11% <ø> (ø)

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