Allow passing query string parameters when proxying requests for Flag endpoints

[tom-scott]

Following on from #477 - I'd like to extend the functionality of api_extra_query_params to the Flag endpoints to allow for non-functional requirements when proxying a request to Mixpanel via infrastructure owned by a customer

Keep with the existing implementation for api_extra_query_params as exists on the observe and engage endpoints for the Flags endpoints.

[tom-scott]

Happy to raise a PR for this issue