Added interceptor fns als replacement for interceptors

[dbeuchler]

Migrated legacy class-based API interceptor into two, single-responsibility Angular functional Interceptors (HttpInterceptorFn).

Additionally, the old class-based components have been explicitly marked as @deprecated.

• Split the ApiInterceptor into languageInterceptor (adds localization headers) and apiErrorInterceptor (handles global error logging).
• emoved the need for withInterceptorsFromDi(), explicit class provider injection, and manual multi-provider arrays (API_INTERCEPTOR_PROVIDER) in app.config.ts.
• Functional interceptors generate less boilerplate code, compile to smaller chunk sizes, and allow for better framework tree-shaking.

Sample:

import { languageInterceptor, apiErrorInterceptor } from './api/api.interceptors';

export const appConfig: ApplicationConfig = {
  providers: [
    provideRouter(routes),
    provideHttpClient(
      withInterceptors([languageInterceptor, apiErrorInterceptor])
    ),
  ],
};