Introduce generic top level handler

[lukehinds]

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)

[lukehinds]

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.