[RFC] Stride multithreading | Separate logic and windowing

Is your feature request related to a problem? Please describe.
Game core must be independet with window. If it will be independent its easy to beals windowless game. I.e. game server.
So you can write once use as you want. You dont need to write same logic again. Also with this feature will help in #1485.
If core will be separated it's easy to made concurency collections / pipes in places where window send events in core and core renders to window.

Describe the solution you'd like
Made core independent. Add RunCoreAsync() which will configure and run core without renderers / window. Add RunAsync() which will configure window / rendering and create em after start.

In fact, it would be cool to create a HostBuilder for Stride. #1487

Here are some questions that can aid in the description of the feature request:

What are the usecases?

• Run raw game login without any rendering. Calculate physics process or game server

What kind of options/settings are expected/wanted?

• separate loginc and rendering with input system. suggested methods

What would the (pseudo) code look like?

• idk. there can be large work. First need to define all places where logic and window acting. separate em.

Is the feature affecting behaviour in the editor?

• No. But game with this can be run in editor directly. Just bind editor scene and run game logic. Editor view will be renderer for us (game window)

Is there a component or datastructure?

• architecture

what kind of documentation is needed or needs to be updated?

• write new docs about how to build windowles and run just core

Should there be an example project?

• one project will be enough. But when networking comes to Stride

Start by mapping where core logic, windowing, rendering, and input currently interact, then review the related discussions in #1485 and #1487. Define the scope for RunCoreAsync() and RunAsync(); done should include a windowless core/server path, a separate rendering path, and documentation or an example showing how to build a windowless game.