feat: Data model for storing App Details and data model for steps

Details:
1. Data model for storing App Details (the agentic system)
As we move towards LLM as Judge metrics, we see that some of these metrics need information about the Agentic system that was used for inferencing. We add a data model to capture that.

2. Data model for Steps
We refine the concept of intermediate data. Previously it stored data in the form of a multiple lists, thereby losing out on the chronological information. This information is needed for some of the metrics. So we refine the concept of intermediate data as series of logical steps that an Agent Take.

PiperOrigin-RevId: 811122784

tests/unittests/evaluation/test_local_eval_service.py

@@ -429,7 +429,6 @@ async def test_mcp_stdio_agent_no_runtime_error():
                 user_content=genai_types.Content(
                     parts=[genai_types.Part(text="List directory contents")]
                 ),
-                expected_response="",
             )
         ],
     )