Extend RealtimeAgent class with model param

### Please read this first

- **Have you read the docs?**[Agents SDK docs](https://openai.github.io/openai-agents-python/)
- Yes
- **Have you searched for related issues?** Others may have had similar requests
- Yes

### Describe the feature
Add model param to RealtimeAgent class.

###What will this give?
It will be possible to set up a smarter model for an agent with specific tasks
