enable use of image proxies

Hi,

I love this great app. Also thumbs up 👍🏼 for you fast reaction to the security incident fixed in the last release.

As to my personal understanding this app downloads images straight from their (external) sources it would be a great improvement in terms of UX to me personal to be able to use an image proxy. (say `images.weserv.nl` or `external.duckduckgo.com`, or a self hosted server etc. ...).

That would increase privacy and might also decrease bandwidth usage. (say by using `webp` output instead of original `png` or `jpg`).

It should only take an additional user preference and a format string in [`DownloadImageHandler.java`](https://github.com/nextcloud/news-android/blob/d4ddf854107cde9d7e3efaa62f7057c12713f971/News-Android-App/src/main/java/de/luhmer/owncloudnewsreader/async_tasks/DownloadImageHandler.java#L50).

The user would enter e.g. `https://weserv.images.nl?url=%s&output=webp`.

Looking forward to your feedback.

Regards, Petra