Agent response format (#660)

# What does this PR do?

Add response format for agents structured output.

- [ ] Using structured output for agents (interior_design app as an
example) (#issue)
https://github.com/meta-llama/llama-stack-apps/issues/122


## Test Plan
E2E test plan with llama-stack-apps interior_design

Please describe:
 Test ran: 

 - provide instructions so it can be reproduced.
 Start your distro:
llama stack run llama_stack/templates/fireworks/run.yaml --env
FIREWORKS_API_KEY=<API_KEY>
 
Run api test:
```PYTHONPATH=. python examples/interior_design_assistant/api.py localhost 5000 examples/interior_design_assistant/resources/documents/ examples/interior_design_assistant/resources/images/fireplaces```


## Sources
Results: 
https://github.com/meta-llama/llama-stack-client-python/pull/72

## Before submitting

- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [ ] Ran pre-commit to handle lint / formatting issues.
- [x] Read the [contributor guideline](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md),
      Pull Request section?
- [ ] Updated relevant documentation.
- [ ] Wrote necessary unit or integration tests.

llama_stack/apis/agents/agents.py

@@ -165,6 +165,7 @@ class AgentConfig(AgentConfigCommon):
     model: str
     instructions: str
     enable_session_persistence: bool
+    response_format: Optional[ResponseFormat] = None
 
 
 class AgentConfigOverridablePerTurn(AgentConfigCommon):