Subscribe multiple topics with a single subscription

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📔 Description

Subscribers can now subscribe for multiple Threat Bus topics by sending a single Subscription message. That is of advantage to the previous implementation, because subscribers now only get to deal with a single p2p_topic for their subscription instead of one p2p_topic per subscribed Threat Bus topic.

📝 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 integration tests
• Use the zmq_sender and zmq_receiver in tests/utils for interactive testing

[rolandpeelen]

I went through the code and it looks good from that perspective 🚀 . Did not run this though.