Support for local proxy - e.g. for Orbot (Tor)

### Actual behaviour
-Tell us what happens
The nextcloud app can't connect to .onion nextcloud-servers

### Expected behaviour
-Tell us what should happen
There should be an option (like many other apps do have, e.g. Privacy Browser, Davdroid, ...) do route traffic through a local proxy (e.g. localhost:8118). Orbot is one of the android proxies that enables access to Tor.

### Steps to reproduce
1. Enter a valid .onion nextcloud server address - it's not recognized
2. There is no option to enable a local proxy either

### Environment data
Android version: 5.1

Device model: Fairphone

Stock or customized system: Stock

Nextcloud app version: 1.4.0, F-Droid

Nextcloud server version: 11.0.1