Adding custom methods to proxy services

[tonisostrat]

Consider the following service which is backed by a standard "rest" type datasource:

export namespace CustomServiceBindings {
  export const CUSTOM_SERVICE = BindingKey.create<CustomService>('services.custom);
}

interface CustomService {
  foo: () => Promise<string[]>;
}

export class CustomServiceProvider implements Provider<CustomService> {
  value(): Promise<CustomService> {
    return getService(this.dataSource);
  }
}

I would like to add a new method to this service, bar: () => Promise<string>; which in essence calls foo(), takes the value, transforms it (string[] -> string) and then returns it. What would be the "correct" way to go about doing that, if possible at all?