SSR API

## Requirements

We need to add out of the box support for Server Side Rendering in BlueRain. This may require some modification in BlueRain as well as `bluerain-cli-web`.

- BlueRain should have a helper method or property to check if the app is running in SSR mode
- Each plugin may add SSR support by hooking into SSR specific hook
- We need to have a way to access SSR context/state which server can use after boot. A use case of this can be router context to return correct HTTP status code from the server.
- How add custom auth on the server side, with cookie or tokens, etc? We may need to have server specific hooks inside or outside BlueRain (i.e. adding custom express middlewares).
- Explore to add support for the .ssr files just like we have platform specific files (e.g. *.web.ts, *.iso.ts, *.android.ts)

Start by reading the BlueRain core and bluerain-cli-web architecture, focusing on existing platform-specific files such as *.web.ts and *.iso.ts. Define how SSR mode detection, plugin hooks, server context, authentication middleware, and *.ssr files should fit together. Done means the SSR API and integration boundaries are specified and supported across both components.

Written by the indexing model from the issue text.