Allow to opt out of caching for certain requests

[mattiapomelli]

Caching is awesome to save on tokens, but there are few scenarios where someone might want to opt out of it.

E.g. allowing to re-generate a response if the user doesn't like it. In this case the prompt is the same as a previous request (so the response would be taken from the cache) but we actually want to obtain a fresh new result.

I would then suggest to allow to opt out of caching on a request basis. Maybe using a specific header for this?