Combine the computed volume keys with the inherited storage pool keys (from Incus)

[MusicDin]

Cherry-picked from lxc/terraform-provider-incus#68

Simple example that was causing an issue:

resource "lxd_storage_pool" "pool" {
  name   = "tf-pool"
  driver = "zfs"

  config = {
    "volume.zfs.block_mode" = false
  }
}

resource "lxd_volume" "vol" {
  name = "tf-vol"
  pool = lxd_storage_pool.pool.name
}

$ terraform apply

│ Error: Provider produced inconsistent result after apply
│ 
│ When applying changes to lxd_volume.vol, provider "provider[\"registry.terraform.io/terraform-lxd/lxd\"]" produced an unexpected new value: .config: new element
│ "zfs.block_mode" has appeared.
│ 
│ This is a bug in the provider, which should be reported in the provider's own issue tracker.