Agent response format

[hanzlfs]

What does this PR do?

Add response format for agents structured output.

• Using structured output for agents (interior_design app as an example) (#issue) Use structured output for interior_design app  llama-stack-apps#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:
meta-llama/llama-stack-client-python#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.
• Read the contributor guideline,
  Pull Request section?
• Updated relevant documentation.
• Wrote necessary unit or integration tests.

[yanxi0830]

Thanks! Could you add an integration test for this in https://github.com/meta-llama/llama-stack/blob/main/tests/client-sdk/agents/test_agents.py?