Add optional corepackage custom client-side files

[mrmanlikesbt]

This PR aims to simplify source code modding so you don't have to run a command on a custom dedicated server or manually install client-side files.

How it works is when you switch your corepackage it will check the new one for a CustomFiles ContentFile, if one is found the player is prompted asking if they want to install the client files. If the player says yes, the files are installed.

A sample mod that changes the unstable main menu background and the warning texts for the installation prompt are attached.

ClientFilesWarningText.zip Test Corepackage.zip