feat: add branch configuration option for rwserver

This commit adds a new configuration option `branch` to the `_default.cfg` file in the `rwserver` directory. The default value is set to "unity". This allows users to specify a specific branch for SteamCMD when installing or updating the server.

lgsm/config-default/config-lgsm/rwserver/_default.cfg

@@ -115,7 +115,7 @@ sleeptime="0.5"
 appid="339010"
 steamcmdforcewindows="no"
 # SteamCMD Branch | https://docs.linuxgsm.com/steamcmd/branch
-branch=""
+branch="unity"
 betapassword=""
 # Master Server | https://docs.linuxgsm.com/steamcmd/steam-master-server
 steammaster="false"