Adjust naming in example plugin

[dominiklohmann]

📔 Description

Per our vote within the team, this renames the functions and classes in the example plugin accordingly and moves it into the vast::plugins namespace as it's a bundled plugin.

📝 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

n/t