Add models.openai and tools.langchain namespaces

[lspinheiro]

Why are these changes needed?

Similar to agents and code executors, moving openai models to its own namespace.

Related issue number

Related to #4408

Checks

• [ ] I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/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.