feat: gemma (text)

This pull request introduces a new function for interacting with Google's Gemma models via the Gemini API, along with the necessary documentation and export updates. The main focus is to provide users with an easy way to generate text using the Gemma models from R. New Gemma API integration: * Added a new function `gemma` in `R/gemma.R` to generate text using the Gemma models through the Gemini API. The function includes parameter validation, API key handling, error reporting, and returns generated text. * Created documentation for the `gemma` function in `man/gemma.Rd`, detailing its usage, parameters, return value, and examples. * Exported the `gemma` function in the `NAMESPACE` file to make it available to package users. Versioning: * Bumped the package version from 0.16.2 to 0.16.3 in `DESCRIPTION` to reflect the new feature addition.