Add a responseMimeType field to the Gemini model

[AliakseiBaryliuk-E]

The EPAM DIAL system currently lacks support for the responseMimeType field in its Gemini model implementation. This field is critical for specifying the MIME type of the response content, which is essential for clients to correctly interpret and handle the data received from the model. By not including this feature, EPAM DIAL limits the flexibility and usability of the Gemini model, as it doesn't fully adhere to the specifications outlined in the official Google Cloud documentation. Incorporating the responseMimeType field would greatly enhance the system's capability to interact with the Gemini model as intended, ensuring better compatibility and a more robust user experience. Link - https://cloud.google.com/vertex-ai/generative-ai/docs/model-reference/gemini