[FDroidRepoBridge] Simplify json retrieval (#4063)

* [FDroidRepoBridge] Simplify json retrieval

I looked into avoiding the writing-to-file and then reading-from-file altogether. Using a special file path that leaves the data in memory probably wouldn't work. But I'm unsure why we use the `index-v1.jar` file altogether.
The main F-Droid repo [lists](https://f-droid.org/en/docs/All_our_APIs/#the-repo-index) not only `index-v1.jar` (which only makes sense if we were to use the contained signature, which we don't), but also `index-v1.json` and `index-v2.json`. These json files can be fetched with `getContents`, optionally cached, and directly fed into `Json::decode` without using a temporary file. The HTTP transfer encoding can compress the file to a similar degree the jar (=zip) can. That's exactly what this commit uses.

Now the question is whether all the F-Droid repositories out there have this file. I went through the whole [list of known repositories](https://forum.f-droid.org/t/known-repositories/721) and only one repo misses the `index-v1.json` file: [Bromite](https://fdroid.bromite.org/fdroid/repo/index-v1.json). Under these circumstances we can depend on the availability of the `index-v1.json` file.

Closes #4062

* [FDroidRepoBridge] Cleanup not requiring Zip

With the last commit 1152386678151aeafd984061d34248023378bf64, the zip
extension is not required anymore. Don't fail if it's not available.

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