Add ExtraFields policy

schemas/__private/policies/extra_fields.py

+from __future__ import annotations
+
+from dataclasses import dataclass
+
+from pydantic import Extra
+
+from schemas.__private.policies.policy import Policy
+from schemas.__private.preconfigured_base_model import PreconfiguredBaseModel
+
+
+@dataclass
+class ExtraFields(Policy):
+    policy: Extra
+
+    def apply(self) -> None:
+        PreconfiguredBaseModel.Config.extra = self.policy

schemas/policies.py

 from __future__ import annotations
 
+from schemas.__private.policies.extra_fields import ExtraFields
 from schemas.__private.policies.policy import set_policies
 
-__all__ = ["set_policies"]
+__all__ = ["set_policies", "ExtraFields"]