Add artifact support for HCP Packer registry

[optik-aper]

Description

Working off of the official documentation (https://www.packer.io/docs/plugins/hcp-support). This should now allow utilization of the hcp_packer_registry block (https://www.packer.io/docs/templates/hcl_templates/blocks/build/hcp_packer_registry) and support adding Vultr snapshots to the HCP Packer registry.

Related Issues

#189

Testing

• configure HCP packer registry (https://learn.hashicorp.com/tutorials/packer/hcp-push-image-metadata?in=packer/hcp-get-started)
• build the plugin
• put this config in the working directory where the plugin binary is located

packer {
  required_plugins {
  }
}

build {
  hcp_packer_registry {
    bucket_name = "test-almalinux-3"
    description = "some description"
    bucket_labels = {
      "owner" = "platform-team"
    }

    build_labels = {
      "build-time"   = timestamp()
      "build-source" = basename(path.cwd)
    }
  }

  sources = [
    "source.vultr.almalinux",
  ]

  provisioner "shell" {
    inline = [
      "echo hello",
    ]
  }
}

variable "vultr_api_key" {
  type    = string
  default = "${env("VULTR_API_KEY")}"
}

source "vultr" "almalinux" {
  api_key              = "${var.vultr_api_key}"
  os_id                = "452"
  plan_id              = "vc2-1c-1gb"
  region_id            = "lax"
  snapshot_description = "testing"
  state_timeout        = "10m"
  ssh_username         = "root"
}

• run packer build <CONFIG_FILE> and wait for it to complete
• check the Packer registry for the results (https://portal.cloud.hashicorp.com/services/packer)

Checklist:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you linted your code locally prior to submission?
• Have you successfully ran tests with your changes locally?

[ddymko]

lgtm