Support non pydantic models as accumulators

[seriousben]

Currently when using a list or other non pydantic models as an accumulator, it returns an error AttributeError: 'list' object has no attribute model_validate.

Having normal python types like a list, string, dict, ... as an accumulator should be supported.

Example that is not supported

@indexify_function(
    accumulate=list(),
)
def accumulate(acc: List[str], data: str) -> List[str]:
  ...