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

[dr-vij]

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

[CodeSmile-0000011110110111]

Related pull request: #3035