Rewrite MISP plugin to convert to/from STIX-2 Indicators and Sightings

[0snap]

📔 Description

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

• Convert MISP attributes to STIX-2 Indicators
• Convert STIX-2 Sightings to MISP sightings
• New message-passing integration test
• Updated unit tests

📝 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

• Run the unit-tests (make dev-mode && make unit-tests)
• Fire up a local MISP or connect to our testbed and start Threat Bus using this branch
• Click some attributes in the MISP web view
• Send some sightings using our test utils

[lgtm-com]

This pull request introduces 2 alerts when merging 9008df7 into dd8c889 - view on LGTM.com

new alerts:

• 2 for Unused import

[tobim]

Good progress. The unit tests are looking great! I think we should also add a check for type mismatches, like what happens if to_ids or deleted contain a string?

[tobim]

I tested this and verified that it works locally. Code changes look good!