fix: use valid pydantic field name

[mtache]

The following change in pydantic breaks jsf: pydantic/pydantic#6797
It is part of pydantic 2.1
Partial traceback:

File jsf/schema_types/base.py:15
     11 class ProviderNotSetException(Exception):
     12     ...
---> 15 class BaseSchema(BaseModel):
     17     type: Optional[Union[str, List[str]]] = None

NameError: Fields must not use names with leading underscores; e.g., use 'schema' instead of '_schema'.

Renamed invalid '_schema' field name to 'json_schema'.

[ghandic]

Fixed in #77