Custom Sync Server Option (Flag or Setting rather than Command Line Arg)

[NicholasFlamy]

This is a replacement for the now-hidden #12314

Description

The only way to set a custom sync server URL is with a command line argument which is not a proper solution, but rather a feature for developer testing. (Including unreliability in the case of a crash and self-restart it won't be applied.) It is also not a solution because on non-rooted Android you have to enable "Enable command line on non-rooted devices" which makes an annoying popup every launch/New Tab opened. iOS doesn't even have a way to use command-line arguments.

So, I propose either a setting within the sync settings to change the sync server URL or a flag. @Fmstrat (https://github.com/Fmstrat/brave-core/tree/feature/custom-flag-sync-server) had a supposedly working solution, one with a GUI option and one with a flag. I have been attempting to create a build with a flag for this but have not quite gotten there. My progress: https://github.com/flamy-brave