Introduce select / replace / extend operators

[dominiklohmann]

This adds the select, replace, and extend pipeline operators. The first is a renamed put, because we abandoned plans to integrate the replace and extend functionality into it. replace is now more powerful and can replace multiple fields in one go. extend is new and allows for adding new fields at the end of a record.

📝 Checklist

• All user-facing changes have changelog entries.
• The changes are reflected on vast.io, if necessary.
• The PR description contains instructions for the reviewer, if necessary.

🎯 Review Instructions

Commit-by-commit. Read the docs carefully.

[dispanser]

Looks good.