LocalMultiplayerAgent should validate StartGameCommand

[dgkanatsios]

LocalMultiplayerAgent should validate StartGameCommand both for processes/containers on Windows/Linux.

1. It should check if the StartGameCommand starts with the MountPath (for container mode).
2. It should check if the executable/script of the StartGameCommand exists in the assets .zip file

[valexao]

The second goal to check if the executable/script exists in the assets .zip file is still open. I believe that a solution could be to use ZipArchive's .GetEntry() to search for the specific file. There are many cases where users will have other command line instructions in their start game command, so parsing the start game command is not straight forward. These changes would take place in MultiplayerSettingsValidator.cs in the isValid() method.