Enable type extractors to support user defined types

[tobim]

📔 Description

This brings back the possibility to run vast count ":port == 80".

📝 Checklist

• 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.
• Add an integration test.

🎯 Review Instructions

By commit.

[mavam]

Nice work!

I looked at the diff in detail and the implementation checks out. Nothing needs to be changed. I couldn't give it a spin locally right now, but it looks really good.

If somebody else could check out the branch to validate the hands-on behavior, that'd be great. The code is in good shape.

[dominiklohmann]

This works nicely.

Needs a rebase and the docs need to be written and merged before this PR gets merged.