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

首先檢視目前的 Unity Transport，以及 issue 中描述的 WebGL 主機模式限制。比較提議的 fake host 模式與自訂離線 Transport，接著定義在沒有網際網路連線的情況下，單一客戶端/伺服器實作應如何同時支援單人遊戲與多人遊戲。完成一個已達成共識的設計，並讓 WebGL 與其他平台上的單人遊戲主機模式正常運作，即表示完成。