Introduce a non-global Vertex AI client

Currently, vertexai.init() sets global variables which is a limit in a multithreaded environment when you want to serve different models to the corresponding available regions. Having an instantiable client (as in your nodejs [client](https://cloud.google.com/vertex-ai/generative-ai/docs/reference/nodejs/latest/vertexai/vertexai) would solve the problem)

从入口点 vertexai.init() 开始，将其全局状态行为与关联的 Node.js Vertex AI 客户端进行比较。定义可实例化 Python 客户端的作用域，并验证不同的客户端可以在不同区域为不同模型提供服务，且不共享配置。

由索引模型根据 Issue 内容生成。