feat(models): Added AssertModel

AsserrtModel needs for checking answer
class AssertModel:
    type: str            # What to check: "status" / "json" / "header"
    path: str            # Where to look: "$.user.id"
    operator: str        # How to check: "equals" / "exists" / "gt"
    value: Any           # What to check against: 200 / true

models/itm.py

@@ -14,6 +14,13 @@ class ExtractModel(BaseModel):
     source: str = "body"
 
 
+class AssertModel(BaseModel):
+    type: str
+    path: str | None = None
+    operator: str = "equals"
+    value: Any = None
+
+
 class StepModel(BaseModel):
     name: str
     request: RequestModel