Introduce generic top level handler

Right now we bind providers directly to an provider interface, but this may be problematic when we have more than one provider wanting access to the same endpoint (e.g. /chat/completions)

Upon discussion we will use the $provider name in the URL .e.g. localhost:8989/openai/chat/completions, we can then route accordingly within codegate.

Specify the provider in the URL to properly route traffic