fix(forge/db): Fix persistence of Step.additional_input and Step.additional_output

- Fixed the persistence issue of additional_input and additional_output in the Step class in `forge.sdk`. The additional_input and additional_output attributes were not typed and initialized properly.

autogpts/forge/forge/sdk/model.py

@@ -113,7 +113,7 @@ class StepRequestBody(BaseModel):
         description="Input prompt for the step.",
         example="Washington",
     )
-    additional_input: Optional[dict] = {}
+    additional_input: Optional[dict] = None
 
 
 class Status(Enum):
@@ -156,7 +156,7 @@ class Step(StepRequestBody):
         description="Output of the task step.",
         example="I am going to use the write_to_file command and write Washington to a file called output.txt <write_to_file('output.txt', 'Washington')",
     )
-    additional_output: Optional[StepOutput] = Field(default_factory=dict)
+    additional_output: Optional[dict] = None
     artifacts: Optional[List[Artifact]] = Field(
         [], description="A list of artifacts that the step has produced."
     )