feat(models): Add StepModel

StepModel is model for 1 step in all test.
class StepModel:
    name: str                    # Step name
    request: RequestModel        # What to send
    extract: list[ExtractModel]  # What to extract from the response
    assert: list[AssertModel]    # What to verify

models/itm.py

@@ -1,3 +1,10 @@
+class StepModel(BaseModel):
+    name: str
+    request: RequestModel
+    extract: list[ExtractModel] = []
+    assert_: list[AssertModel] = []
+
+
 class TestModel(BaseModel):
     name: str = "Unnamed Test"
     base_url: str = ""