add `baseURL` for the proxy

[nathanwhy]

Description of the change

Modified the url computed property within the GenerativeAIRequest extension to use the provided baseURL if available. If baseURL is nil, it defaults to GenerativeAISwift.baseURL.

Motivation

The primary motivation for this change is to enable routing of requests through a custom server proxy. By introducing an optional baseURL, the application gains the ability to direct API calls through an intermediary server.

Type of change

Feature request

Checklist

• [x] I have performed a self-review of my code.
• [x] I have added detailed comments to my code where applicable.
• [x] I have verified that my change does not break existing code.
• [x] My PR is based on the latest changes of the main branch (if unsure, please run git pull --rebase upstream main).
• [x] I am familiar with the Google Style Guide for the language I have coded in.
• [x] I have read through the Contributing Guide and signed the Contributor License Agreement.

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