Introduce apstra_datacenter_routing_zone_loopback_addresses resource

[chrismarget-j]

This PR introduces the apstra_datacenter_routing_zone_loopback_addresses resource.

It's a vaguely imperative resource in the sense that it doesn't create anything, it merely assigns (or clears) address values from fields which already exist and may already contain values.

This resource relies heavily on a private data struct which records system IDs where values were previously written by the Create() and Update() methods. The record looks like this:

{
  "GX652VT5ZkH5JOg1Jw": {
    "has_ipv4": false,
    "has_ipv6": true
  },
  "UXAnfRFHDnkybwHAbw": {
    "has_ipv4": true,
    "has_ipv6": false
  }
}

The various methods make use of these values when determining which interfaces and addresses are relevant to their work:

• Read() discards values not previously assigned by this resource
• Update() clears values previously assigned but not part of the current plan
• Delete() clears all previously assigned values

The Request() function in this resource is different from most for a couple of reasons:

• It takes a client object as input to make an API call which maps system IDs (what we collect from the user) to interface IDs (what we send to the API)
• It returns the private struct above based on the API payload it has prepared.

Closes #981