Improve authentication flow with auth token

Add token exchange. Create token on server and use as a cookie on the client.

We should use this as a default and issue a deprecation warning to anyone using the old methods.

See: https://github.com/cdr/code-server/pull/3422#issuecomment-853297900

**Edit**: VS Code web already has token auth that we disable, could we make use of this? Then it should be pretty easy.

首先查看所链接 pull request 的讨论，以及 VS Code web 现有的令牌身份验证。然后跟踪当前服务器和客户端的身份验证方法，再定义令牌交换和 Cookie 流程。完成的标准是令牌身份验证成为默认方式，旧方法发出弃用警告，并且现有的身份验证行为仍然有覆盖。

由索引模型根据 Issue 内容生成。