Add Zeek Broker reader plugin

[mavam]

📔 Description

This PR adds a new broker reader plugin, enabling VAST to be true native
Logger node. This approach takes a slightly different path than the usual way
of writing a Zeek plugin and implementing one's own WriterBackend. Here are
the pros and cons of this method.

Pros:

• Easy rollout: works out-of-the-box with Zeek, just load a script and log
  remotely
• Works seamless with the Cluster framework: VAST is a first-class logger node
• Performance: no need to "log twice," once to the Logger node and then
  parsing/repacking the data again to ship it to VAST

Cons:

• Does not use a standard API and reverse-engineered binary wire format
• No support for Zeek logging filters, if used

Since filtering can also be implemented easily downstream in VAST, we find that
the pros dominate this method.

📝 Checklist

• Replace poll with get(num, timeout)
• All user-facing changes have changelog entries.
• The changes are reflected on docs.tenzir.com/vast, if necessary.
• The PR description contains instructions for the reviewer, if necessary.

🎯 Review Instructions

File-by-file.

[dominiklohmann]

LGTM. I just have some smaller things, plus this needs CI to run through. Let's get this in and then test this in our E2E setup outside of this repository.