plugin architecture: Zeek packages

[mmguero]

@mmguero cloned issue idaholab/Malcolm#582 on 2024-10-01:

This is a sub-issue of Malcolm "plugin architecture" #399

What if users want to drop in a custom zeek package as a Malcolm plugin?

There may be some overlap with plugin architecture: custom rules/policy/scripts/config/etc. #581, although in this issue I'm talking more of an actual Zeek package itself to be installed with zkg (for example, a new parser) rather than just custom scripting.