Adding SA key authentication to gemini model initialization

[BergerKing333]

This PR adds an option to initialize a gemini model based on a Google Cloud platform project's SA key using the project name + location. This is helpful for situations when a user might have access to a company GCP project and SA key, but without access to a Gemini API key-- this is the situation I was in, and this fix let me skip the hoops of requesting the tech team to provision me a specific API key.
Achieving this is simple; when calling a gemini model, a user now has an option of 2 additional parameters; location and project_id. If the API key is not set, the code now attempts to create a genai instance using an SA key before throwing the "No API key detected" exception. The ReadMe and in-line documentation have been updated to reflect this change as well.

Fixes #58

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