Implement overrides for sizes and locations

[goncalo-rodrigues]

Something like:

message VirtualNetworkArgs {
  ...
 
 message AwsOverrides {
    string vm_size = 1;
    string location = 2;
  }
 message AzureOverrides {
    string vm_size = 1;
    string location = 2;
  }
  AwsOverrides aws_overrides = 1;
  AzureOverrides azure_overrides = 2;
}

Which will then be used in the translation logic to override cloud-specific values.