Hardcoded `System.getenv()` calls prevent programmatic configuration of parameters

The Google ADK currently relies strictly on `System.getenv()` to read critical configuration parameters, specifically `GOOGLE_GENAI_USE_VERTEXAI`, `GOOGLE_CLOUD_PROJECT`, and `GOOGLE_CLOUD_LOCATION`.

This creates a significant architectural friction point for modern Java applications, such as Spring Boot on Java 17+. In these environments, configurations are managed internally via files like `application.yaml` rather than static OS-level environment variables. Because Java's `System.getenv()` reads an immutable OS map, we are completely blocked from programmatically injecting or wiring these values from our application's configuration at runtime.

This forces developers to rely on external CI/CD deployment wrappers or unsafe JVM reflection hacks just to pass standard configuration values into the SDK.