Add data source

May be a good idea to get info from externally managed sources

Rough example:

```
# Resources to create here
provider "ipam" {
alias = "ours"

file = "ours.ipam.json"
}

# Data source
provider "ipam" {
alias = "theirs"

file = "/mnt/nfs/theirs.ipam.json"
}
```

Keeping in mind, it is only reasonable to obtain unknown stuff, e.g. child CIDRs list