Add gemini as a supported model provider

[dangazineu]

Both GPT and Reasoning model families should map to the following models:

• Small: "gemini-2.5-flash-preview-05-20"
• Large: "gemini-2.5-pro-preview-06-05"

Use GEMINI_API_KEY env variable for the api key.

[dangazineu]

This was resolved by #25