feat(core): Add module overrides system

[Ehesp]

This PR creates a feature which enables module overrides.

Since we're supporting Angular components via AngularFire, which wraps Firebase exports in zones, our own internal core implementation does not know about these, and thus usage from Angular will call the Firebase exports and potentially cause issues with reactivity, SSR etc.

This PR will allow the Angular implementation to set a module implementation which core will use if available, otherwise it'll fallback to firebase/auth.

import { setAuthOverrideModule } from '@firebase-ui/core/overrides'; // Export TODO

// ... somewhere in the app
setAuthOverrideModule(ui, () => import('@angular/fire/auth'));