feat: deprecate assigning function to schema in favor of api-party:extend hook

[mattmess1221]

🔗 Linked issue

#87

❓ Type of change

• 📖 Documentation (updates to the documentation, readme or JSDoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This change allows developers to easily hook into the endpoints and update properties like the schema.

Implements #87

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

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[johannschopplich]

Just to clarify: Let's review and merge this one here first before getting #89 ready, right?