feat: add singleton app ref

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Other information: The AAppContext is not able to be shared across different instances of AApp. With the move to micro-frontends, we will need to have the host app and remote micro-frontends define AApp separately (or not at all). Since AApp is traditionaly used only once on a page, we can add a window reference and fallback to that in the case that AApp is not defined in a remote micro-frontend.

cisco-sbg-ui / atomic-react

feat: add singleton app ref #1274