provider/aws: Allow defining network interfaces in aws_instance

[freimer]

Implement aws network_interface for aws_instance type. We can now create instances with multiple network interfaces, like so:

resource "aws_instance" "instanceName" {
  network_interface {
    network_interface_id = "${element(aws_network_interface.mgmt.*.id, count.index)}"
    device_index = 1
  }
  network_interface {
    network_interface_id = "${element(aws_network_interface.public.*.id, count.index)}"
    device_index = 0
  }
  network_interface {
    network_interface_id = "${element(aws_network_interface.private.*.id, count.index)}"
    device_index = 2
  }
}

I know I didn't do everything, such as documentation updated, so please tell me what I need. I also don't officially know Go, so there's that.
It works though ;-)

[grubernaut]

Closing via #12933

[ghost]

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