SSR: Preload Feature App modules before hydrating in the client

[clebert]

After rendering Feature Apps on the server the JS files of the Feature Apps that were rendered on the server need to be loaded before hydrating.

• The integrator collects Feature App URLs for hydration via the FeatureAppLoader who knows which Feature Apps have been rendered on the server. The FeatureAppLoader adds the URLs with a addUrlForHydration function that is provided via the FeatureHubContext.
• The set of URLs is serialized into the HTML document by the integrator.
• On the client the integrator loads the URLs from the HTML document and calls the preloadFeatureApp method of the FeatureAppManager for all URLs before hydrating.
• We need to document the known edge case, maybe in the FAQ.
  • When a server-rendered Feature App view requires additional JS chunks in the client (via code splitting), then we can't guarantee that hydrating works without visual impact.
• Not appearing anymore is "Warning: Did not expect server HTML to contain a..."

Tasks

• add addUrlForHydration to FeatureHubContextValue and InternalFeatureAppLoader interface feat(react): add addUrlForHydration and integrate into FeatureAppLoader #320
• use addUrlForHydration in FeatureAppLoader feat(react): add addUrlForHydration and integrate into FeatureAppLoader #320
• integrate preloading (fix hydration) into ssr demo feat(demos): use addUrlForHydration for preloading in ssr demo #322
• documentation: adapt SSR guide docs(website): write preloading section in SSR guide #324
• write JSDOC comments feat(react): add addUrlForHydration and integrate into FeatureAppLoader #320