[native_doc_dartifier] Add Gemini caller Utility

[marshelino-maged]

Creating a utility that calls the Gemini LLM with a given prompt and an optional JSON schema:

• Accepting a prompt with an output JSON schema with descriptions for each field, which will be concatenated with the prompt, ensuring the final output is easily parseable and aligned with structured data.

• Accepting a prompt without an output JSON schema will require some work to handle and parse the output of the LLM to continue working with it.