Added: Operator overloads for Request Usage

[rohanthacker]

Why are these changes needed?

This PR adds the operator overloads for __add__ and __iadd__ for the Request Usage class.

Currently we track usage via the _add_usage function that is part of the open_ai module in autogen-ext. Considering this feature is needed by all model clients adding these methods to the class directly allows for re-use in other classes.

Related issue number

Extends upon the conversation in #4770

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.

[rohanthacker]

@jackgerrits @gziz Hello, I've implement this based upon the conversation in #4770. I've opened this as draft for feedback.
Once discussed I'll be happy update this PR with the updated open_ai client. I've completed this locally and all the tests are passing.