Add optional prompt attribute to Code generation endpoint

As discussed in gitlab-org/gitlab#423522 (closed) we want to send the already pre-made prompt to the model gateway which would be optional and then send directly to the google model.

We still would send the prefix so the model gateway can use the original code for deduplicating potential response and also do the standard cleaning of the response.

So the additional prompt attribute overrides any prompt creation in the model gateway for the code creation endpoint

Seperate issue then for the code completion endpoint so we can do this there too in the future if needed.