[backend] Merged entity should also have all files of the merged entity and at the right path in S3 (#6258)

[aHenryJard]

Proposed changes

• Add a test "example" on merge Artifact, did not cover all use cases with all entities
• Add a s3 method to move file from one entity to another
• update x_opencti_files content on the target entity for the merge
• Remove S3 metadata (as per issue comment): it's not needed anymore since we store metadata in elastic (InternalFile)
• fix x_opencti_file ids on artifact creation

Refactor:

• API_TOKEN to ADMIN_API_TOKEN (in testQuery)

Related issues

Checklist

• [ ] I consider the submitted work as finished
• [ ] I tested the code for its functionality
• [x] 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

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...