This creates a new "custom_master_server.txt" file that can be used to pre-seed a master server list.
This also allows for steam apps to manipulate the master server list in memory.
I.e. This implements:
Steam_Masterserver_Updater::AddMasterServer()
Steam_Masterserver_Updater::RemoveMasterServer()
Steam_Masterserver_Updater::GetNumMasterServers()
Steam_Masterserver_Updater::GetMasterServerAddress()
Signed-off-by: redpolline <11156324-redpolline@users.noreply.gitlab.com>