Add support for reserved private IP in GCP builder

[helldrum]

Description

Add the ability to specify a reserved private IP address when creating GCP instances with Packer. This feature would allow users to assign a specific private IP address to instances during the build process.

Use Case(s)

When building images in environments with strict networking requirements or when IP addresses need to be pre-defined for compliance or architectural reasons, users need to be able to specify a static private IP address for the build instance.

Potential configuration

source "googlecompute" "example" {
  project_id   = "your_project_id"
  zone         = "europe-west1-b"
  machine_type = "e2-micro"
  subnetwork   = "my-subnet"
  reserved_ip  = "10.0.0.10"  # Optional: Specify a reserved private IP
}

Potential References

Add reserved_ip field to the builder configuration structure Implement validation in the Prepare function Modify instance creation to use the specified IP when provided Update documentation to reflect the new feature

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