Merge lists from configuration and CLI

[dominiklohmann]

📔 Description

Before this change, we merged configuration from different sources in a way that instead of merging nested lists, we overwrote them—leading to plugins specified in a configuration file not to load at all when plugins were specified on the command line as well. Now, we merge the lists in order of precedence.

📝 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.

🎯 Review Instructions

File-by-file. Test locally.

Note that changelog entry + docs are yet to be written because I'd like to discuss this first with @tenzir/backend.

[lava]

I didn't look too deeply since this was already reviewed in a depth in a call with @tobim , but from what I can see the code is correct and the review comments were addressed.