feat: support source_snapshot for additional_disks

[anisf]

This PR adds support to specify a source snapshot to use when creating an additional disk

This parameter is already supported from the google_compute_instance_template terraform resource but was not usable from this module

Default behaviour remains unchanged as this param is optional

Here's an example of how to use it :

module "instance_template" {
...
  additional_disks = [
    {
      disk_name       = "disk-0"
      device_name     = "disk-0"
      disk_size_gb    = 10
      disk_type       = "pd-standard"
      auto_delete     = "true"
      boot            = "false"
      disk_labels     = { "foo" : "bar" }
      source_snapshot = "projects/my-project/global/snapshots/my-snapshot"
    },
    {
      disk_name       = "disk-1"
      device_name     = "disk-1"
      disk_size_gb    = 10
      disk_type       = "pd-standard"
      auto_delete     = "true"
      boot            = "false"
      disk_labels     = { "foo" : "bar" }
    }
   ]
...
}

[g-awmalik]

/gcbrun

[hSATAC]

I noticed that the this pull request introduces the optional keyword, a feature available in Terraform v1.3.0. However, the README file indicates that this module is designed for use with Terraform 0.13+. I thought it might be worth bringing this to your attention for potential version compatibility considerations.