Feature request: Data sources

[jsimpso]

It would be great if we could use the provider to reference existing resources as data sources, e.g. referencing a network by name to have another resource act on its IPv4 range:

data "lxd_network" "my_net" {
  name     = "my-net"
  project  = "my-project"
}

resource "example" "example" {
  ip_range = data.lxd_network.my_net.ipv4
}