Add support for source image via URL

[pdesai-ntnx]

/kind feature

Currently to get the image built, we have to upload the source image to Prism Central and use the name/uuid in nutanix.json.
Ideally we should be able to specify a URL like http://cdimage.ubuntu.com/ubuntu-legacy-server/releases/20.04/release/ubuntu-20.04.1-legacy-server-amd64.iso, and use Terraform to pull the image onto PC. This will reduce a step in the build process.

Additionally, if possible add checks if image already exists, so we don't end up creating multiple image pulls, especially when this is ran through automation.

Environment:

• packer-plugin-nutanix version (use packer plugins installed):
• Packer version (use packer version):
• OS (e.g. from /etc/os-release):