Rewrite CIF3 plugin to convert STIX-2 Indicators

[0snap]

📔 Description

Following up on the STIX-2 rewrite of Threat Bus: this PR updates the CIF3 plugin.

• Convert STIX-2 Indicators to CIF3 Indicators

📝 Checklist

• All user-facing changes have changelog entries.
• The changes are reflected on docs.tenzir.com/threatbus, if necessary.
• The PR description contains instructions for the reviewer, if necessary.

🎯 Review Instructions

Code review should be enough as this plugin has seen little attention and there are no unit tests, to begin with.

If you want to test interactively:

• Follow the instructions in the CIF3 plugin's readme to start a CIF3 container and copy the API TOKEN to your Threat Bus config.yaml
• Send Indicators via Threat Bus and confirm they make their way into CIF3 (e.g., see the logs)

[lava]

The docker build of CIF seems to be currently broken, so I was not able to personally test the plugin changes as described in the instructions.

However, the code itself looks good to me, so I'm approving this PR.

[0snap]

I tested this with a local installation of CIF 3 and indicators updates are forwarded to CIF without errors.