[Feature Request] Compatibility of generated Pydantic models with FastAPI for proper OpenAPI Swagger serialization

## Is your feature request related to a problem? Please describe.

I'm currently using betterproto to generate Pydantic models for my FastAPI application. However, FastAPI is unable to properly serialize the object to display it in the OpenAPI Swagger UI. This makes it difficult to understand and interact with the generated API documentation.

## Describe the solution you'd like

I would like betterproto to generate Pydantic models that are compatible with FastAPI, allowing FastAPI to serialize the objects correctly in the OpenAPI Swagger UI. This could involve making any necessary modifications to the generated models, such as adding the appropriate Pydantic schema configuration or validators.

If anyone has any suggestions or ideas on how to implement this feature, I'd be happy to work on it myself and contribute to the project. This feature would greatly improve the user experience for developers using both betterproto and FastAPI, as it would allow for seamless integration of the two technologies.

The issue names no files, tests, or entry points. Start by reproducing the generated-model response in FastAPI and inspecting the resulting OpenAPI Swagger serialization. Done means generated Pydantic models serialize correctly and appear properly in the OpenAPI schema.

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