Support for choosing a custom backup location

**Is your feature request related to a problem? Please describe.**
I have a cloud folder on my android device. I configure some apps to do regular backups into that folder. I would like to do this with AnkiDroid, too, but I don't want to put the main AnkiDroid folder into my cloud folder, since it contains 10k+ (media) files.

**Describe the solution you'd like**
An option for choosing a custom path for the backup folder.

**Describe alternatives you've considered**
Relying on AnkiWeb for backup. Or manually copying the backup files.

**Additional context**
Since AnkiWeb provides some way of backup, it is of less priority. However since local backups are still encouraged (by the AnkiWeb docs/terms), I think it would be a nice option. Also this is only relevant for setups that do not sync to another device regularly (since then that would be an additional form of backup).