Add `source_image` to `disk_attachment`

[adsr]

Add ability to specify a source_image in a disk_attachment.

Relevant excerpt from GCP godocs:

    // SourceImage: The source image to create this disk. When creating a
    // new instance, one of initializeParams.sourceImage or
    // initializeParams.sourceSnapshot or disks.source is required except
    // for local SSD.

[hashicorp-cla]

All committers have signed the CLA.

[adsr]

Here's a barebones template that illustrates what we're going for.

source "googlecompute" "image_extra_disk" {
  project_id = var.gcp_project_id
  zone = var.gcp_zone

  source_image_family = var.source_image_family

  image_name = var.image_name

  disk_attachment {                     
    disk_name = "extra-disk"                                
    device_name = "extra_disk"
    source_image = var.extra_disk_source_image_family
    volume_type = "pd-standard"
  }                   

  image_source_disk = "extra-disk"
}

[adsr]

~I've noticed it's actually creating 3 disks, the boot disk, an extra disk, and then a 3rd disk with the image. Need to fix this up. Converted to draft for now.~

EDIT: Force-pushed to 689fc55. Now creating just 2 just disks as expected.

[adsr]

Ran goimports and gofmt

[nywilken]

Hi @adsr thanks for your help in pushing this enhancement forward. It looks like there were just a few suggestions left to address with a rebase to resolve conflicts.

Are you still interested in pushing this PR forward?

[uriyyo]

Hi @adsr @lbajolet-hashicorp,

We would like to use this feature in our project. Is there smth we can help with? In case we are stuck with this PR, we can open an alternative PR with this feature.

Please, let me know if there is smth we can help with 🙏