Make it easier to extend Activity Vocabulary API with custom activities/objects

[dahlia]

Currently, extending Fedify's Activity Vocabulary API requires either:

1. Manually extending classes and implementing fromJsonLd() for each class
2. Forking the repository and adding YAML files to the codegen system

We should provide a simpler way to extend the Activity Vocabulary API at runtime. Some possible approaches:

• Registry system for custom types
• Plugin system for vocabulary extensions
• Runtime code generation from schema definitions

This would make Fedify more flexible for applications that need custom ActivityPub vocabularies.