Add a key to make fields (columns) nullable

[Ludee]

Summary of the discussion

To describe the table schema in the resource field, we need to add an option for nullable fields. As the schema field is also used for SQL table definitions, the nullable constraint is mandatory.

Type of change (CHANGELOG.md)

Added

• Add key nullable to fields section (columns) (#161)

Workflow checklist

Automation

Closes #96
Closes #123

PR-Assignee

• 🐙 Follow the workflow in CONTRIBUTING.md
• 📝 Update the CHANGELOG.md
• 📙 Update the documentation

Reviewer

• 🐙 Follow the Reviewer Guidelines
• 🐙 Provided feedback and show sufficient appreciation for the work done

[jh-RLI]

LGTM