fix(terraform): fix reference extraction from TupleConsExpr

[nikpivkin]

The AllReferences method of the ssh_keys attribute returns only 1 reference to a block, but must return 2. The attribute expression is TupleConsExpr, which represents a list or set of expressions that can reference another block. This PR fixes this and extracts references from each TupleConsExpr expression.

Sample config:

data "digitalocean_ssh_key" "first" {
  name = "mikhalych_deploy_key"
}

data "digitalocean_ssh_key" "second" {
  name = "d.lukshto-deploy-key"
}

resource "digitalocean_droplet" "this" {
  image              = "ubuntu-20-04-x64"
  name               = "test"
  region             = var.region
  size               = "g-2vcpu-8gb"
  private_networking = true
  ssh_keys = [
    data.digitalocean_ssh_key.first.id,
    data.digitalocean_ssh_key.second.id,
  ]
}