Implement a single player mode (fake host mode) for WebGL

Is your feature request related to a problem? Please describe.
I want to have the ability to implement single player mode for WebGL, but host mode is not available there

Describe the solution you'd like
I want to have a solution that allows me to create fake host mode out of the box, without internet connection to use it for singleplayer mode for all platforms, especially for webgl. It is needed to have 1 client/server implementation for singleplayer and multiplayer games.

Describe alternatives you've considered
To create a custom Transport without internet connection. Or maybe to make host mode for webgl that uses other driver inside current Unity Transport

首先检查当前的 Unity Transport，以及 issue 中描述的 WebGL 主机模式限制。将提议的 fake host 模式与自定义离线 Transport 进行比较，然后确定在没有互联网连接的情况下，单个客户端/服务器实现应如何同时支持单人游戏和多人游戏。达成一致的设计，并使 WebGL 和其他平台上的单人游戏主机模式能够正常工作，即视为完成。