Support specifying `apiVersion` in Gemini API config

[ikechan8370]

Add support for configuring the Gemini API version via the apiVersion field in configuration options, similar to the interface used in the genai library.

Developers should be able to specify a custom API version for the Gemini service when initializing the client. If not set, the default API version should be left blank and use the SDK default.

Reference: chatgpt-plugin#822

---

Example reference from genai:

export interface GoogleGenAIOptions {
  ...
  /** Optional. The API version to use. 
   * If unset, the default API version will be used. 
   */
  apiVersion?: string;
  ...
}

Acceptance Criteria:

• The apiVersion field is supported in the config for Gemini API.
• If apiVersion is unset or blank, the default SDK behavior is to be used.
• Documentation is updated to clarify usage of apiVersion for Gemini API.

See also chatgpt-plugin issue 822 for the motivating scenario.