[Taxii2] Option to pull data from custom property and put in a label

[annoyingapt]

Proposed changes

I have a taxii 2.0 feed that is pushing interesting data in a stix custom property. Currently it is being dropped by OpenCTI. I've added some code (disabled by default) that will allow a user to define the custom property as a variable and save the data as a label. I wasn't sure if there was a better way to do this.

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

Further comments

I did pop the question into slack, just in case I get any interesting responses there.. https://filigran-community.slack.com/archives/C06CC3T5Z37/p1716258546130119