[i18n] Load translations from backend

[sissbruecker]

The client should load translations from the Java backend. This should happen:

• When the client / I18n initializes, loading translations for the initially detected language
• When the user changes the language, loading translations for the newly selected language

The backend should provide either a request handler, or a Hilla endpoint, for loading translations for a specific language. That handler would return the contents of the language specific properties file from the translations resource bundle already used by Flow apps. If there are no translations for the requested language, the backend should return a default set of translations, which should be the contents of the root resource bundle (translations.properties). The handler should somehow indicate to the client whether it was able to resolve translations for the requested language, or whether it returned the default as a fallback. Only if no fallback can be found, the handler should return an error status code, and the client should be able to show an error as well.

For now the client would load all translation strings stored in the translations resource bundle, which could include translation strings that are only used by the backend (email templates, report templates, etc.). A future enhancement could cover splitting translations, so that we only load the translations that are implicitly (automatic detection) or explicitly (developer managed) needed by the client. An alternative could be to configure filters in the backend which keys to send to the client.

[sissbruecker]

PR for adding a request handler for loading translations: vaadin/flow#18680