[Sekoia] Retrieve the list of entity sources

[Lhorus6]

Sekoia provides a list of sources who have reported the information. The information is contained in a custom Sekoia field, named "x_inthreat_sources_refs".

The various sources would have to be imported into the platform.

Proposed changes

• Since STIX does not allow multiple “Author”, we record them in labels, e.g. “source:Name1”, “source:Name2”, ...

Related issues

• [Sekoia] Retrieve the list of entity sources #3176

Checklist

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

[Lhorus6]

Hi @romain-filigran, @helene-nguyen,
It's a modification for a customer. Let me know when we can organize the review to validate the modification. We can do this together if it saves you time.

[flavienSindou]

Some small remarks for optimization but everything seems fine to me. Thanks you for your contribution !