feat: use `numeric` type on fields with `decimal` casts

[ekisu]

Summary

This PR changes the type of fields with decimal casts to numeric.

Closes #1582

Type of change

• [x] New feature (non-breaking change which adds functionality)

Checklist

• [x] Existing tests have been adapted and/or new tests have been added
• [x] Add a CHANGELOG.md entry
• [x] Update the README.md
• [x] Code style has been fixed via composer fix-style