Allow gemma models

Hi, Thank you for your effort on this package. As for several functions in gemini_*.R, the name of the model is coded with prefix "gemini-" as in: `paste0("gemini-", model, ":generateContent")` I would like to suggest that this is changed to: `paste0(model, ":generateContent")` and `function(..., model = "gemini-2.0-flash", ...)` so as to also allow gemma models and other models that do not start with "gemini-". (although I am aware that the name your package is `gemini.R`, but gemma is part of gemini too). Best regards and thanks again for the package.