Ability to select update channel

### Issue Details

Now, in order to try new features in the beta version or check that the nightly version has been fixed, the user needs to find information on where to download it himself, and then manually install it over the current version.

### Proposed solution

Add the ability to select the update channel in the app settings so that the user can install the beta or nightly via the built-in updater (as in AdGuard for Windows).

### Alternative solution

Add information about the beta version to the settings and a link to how user can install it: https://adguard-vpn.com/en/beta.html?platform=windows