[backend] observables values implemented (#8312)

[ValentinBouzinFiligran]

Proposed changes

• observables values key added on indicator properties

Related issues

• Ability to define and implement “Exclusion Lists” opencti#8312

Checklist

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

Further comments

• [backend] observables values implemented (#8312) opencti#8449