No auto-injection of global styles to the embedded components

[mshabarov]

Describe your motivation

Vaadin 25 supports autoInjectGlobalCssImports flag in theme.json that controls the injection of host app styles into embedded components. However, this is deprecated along with the Theme annotation and we need an alternative to control the injection.

Describe the solution you'd like

Instead of adding one more parameter/flag, we can let users to control this by placing CssImport annotation on the right place:

• Common styles in the app and embedded components - add CssImports to the AppShellConfigurator implementing class and all embedded components (WebComponentExporter).
• Application specific styles that only wanted in the application. Add CssImports to the AppShellConfigurator implementing class only and they don't leak into embedded components.
• Embedded specific styles isolated in embedded component. Add CssImports to WebComponentExporter only and these styles would not leak into host app (already implemented in Styling embedded web components using CssImport annotation #22637)

[platosha]

Would make sense to recommend using standard CSS features (CSS shadow parts, custom CSS properties) for styling embedded components, same approach as in Aura, instead of providing a new custom API for a feature being deprecated? This would require less implementation in Flow.