[backend] observables values implemented (#8312)

[ValentinBouzinFiligran]

Proposed changes

• observables values key added on indicator properties

Related issues

• https://github.com/OpenCTI-Platform/opencti/issues/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

• https://github.com/OpenCTI-Platform/opencti/pull/8449