Add structured output

[davorrunje]

Why are these changes needed?

Add structured_format parameter to the ConversibleAgent class and use it to define JSON output format of the agent.

Related issue number

Closes #23

Checks

• I've included any doc changes needed for https://ag2ai.github.io/ag2/. See https://ag2ai.github.io/ag2/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[davorrunje]

We are waiting for @sternakt to push his changes to this branch.

[qingyun-wu]

@ohdearquant could you help review this? Please approve if it reads good to you! Thank you!

[qingyun-wu]

@davorrunje @sternakt It seems meta data of the notebook included in this PR was not updated properly. I made direct changes to the notebook, so we are all good here. Please remember to check meta data in future notebooks. Thank you!

[ohdearquant]

Looks good. Main changes are the response format and oai client, which is a clean straightforward implementation.