feat: Add extension system for custom app question types

[Bahati308]

Add comprehensive extension support allowing custom apps (e.g., AnthroCollect)
to define their own question types, renderers, functions, and schema definitions
without requiring changes to ODE core.

Architecture:

• Formulus: Extension discovery and loading via ExtensionService
• Synkronus: Extension validation during bundle upload
• Formplayer: Runtime dynamic import and registration

Key Features:

• Discovers ext.json files at app-level (/forms/ext.json) and form-level
  (/forms/{formName}/ext.json) with merge precedence
• Validates extension structure, module existence, and renderer references
• Dynamically loads and registers extension renderers at runtime
• Supports extension definitions for JSON Schema $ref resolution
• Graceful error handling with actionable validation messages

Files Added:

• formulus/src/services/ExtensionService.ts
• formulus-formplayer/src/extensionLoader.ts
• EXTENSION_IMPLEMENTATION.md

Files Modified:

• formulus/src/services/index.ts (export ExtensionService)
• formulus/src/components/FormplayerModal.tsx (load and pass extensions)
• formulus/src/webview/FormulusInterfaceDefinition.ts (add ExtensionMetadata)
• formulus-formplayer/src/App.tsx (register extension renderers)
• formulus-formplayer/src/FormulusInterfaceDefinition.ts (add ExtensionMetadata)
• synkronus-cli/pkg/validation/bundle.go (add extension validation)

Constraints Met:

• No app logic in ODE core
• No hard-coded app paths
• No use of eval
• Backwards compatible (extensions optional)
• Declarative only (no JS imports in ExtensionService)

[r0ssing]

Thank you so much @Bahati308. I think this is exactly what we need! 👍

Would it be possible to add a an example of this system to our malaria demo app?
That way we could see if it works end-to-end, by:

• Defining the appropriate ext.json (e.g. a "Mosquito size" question type, with 5 sketches of mosquitoes in different sizes or something)
• Defining the appropriate renderer +tester
• Defining the question in jsonforms schema/ui json

[Mishael-2584]

Nice work @Bahati308 would love to see how this takes shape with the forms we have

[r0ssing]

Solid work! I really think this enables a rich ecosystem of extensions for the custom apps👌😃

[Bahati308]

Solid work! I really think this enables a rich ecosystem of extensions for the custom apps👌😃

thanks @r0ssing , let's go for it.