Migrate to use Cloud-init instead of StartUp-Script for the startup script

[Zurvarian]

We've using Atlantis in GCP and received a notification recently that states that using the Startup Script as part of the VM's metadata is going to be deprecated by July 31, 2026.

The recommended approach to keep things as simple as possible (I.e. avoid migrating to CloudRun) would be to use Cloud-Init to handle the startup script.

To ease clients upgrading to a newer version of this project without major changes I propose to use a combination of WriteFiles + Runcmd where the actual StartUp Script is saved as an executable script into the VM using WriteFiles and then, the said file is executed on the Runcmd module.

Alternatively, it would be possible to embed the startup script as part of the User Script module.

Though, I'm open to alternatives as my main interest is to avoid the deprecation issue.