feat(DUP): add libcluster topology for VerneMQ discovery

Introduce support for VerneMQ replica discovery via libcluster.
This change adds topology definitions for both docker-compose and
Kubernetes environments.
This enables DUP to dynamically connect to all VerneMQ nodes in the
cluster, as the needed step to support doing RPC operations (e.g.
requesting a device disconnection) via Erlang's native message passing.

Signed-off-by: Davide Briani <davide.briani@secomind.com>

Author: davidebriani

apps/astarte_data_updater_plant/lib/astarte_data_updater_plant/config.ex

@@ -265,6 +265,14 @@ defmodule Astarte.DataUpdaterPlant.Config do
           type: :binary,
           default: "app=astarte-data-updater-plant"
 
+  @envdoc "The Endpoint label to use to query Kubernetes to find vernemq instances. Defaults to `app=astarte-vernemq`."
+  app_env :vernemq_clustering_kubernetes_selector,
+          :astarte_data_updater_plant,
+          :vernemq_clustering_kubernetes_selector,
+          os_env: "VERNEMQ_CLUSTERING_KUBERNETES_SELECTOR",
+          type: :binary,
+          default: "app=astarte-vernemq"
+
   @envdoc "The Kubernetes namespace to use when `kubernetes` Erlang clustering strategy is used. Defaults to `astarte`."
   app_env :clustering_kubernetes_namespace,
           :astarte_data_updater_plant,
@@ -514,6 +522,18 @@ defmodule Astarte.DataUpdaterPlant.Config do
               kubernetes_namespace: clustering_kubernetes_namespace!(),
               polling_interval: 10_000
             ]
+          ],
+          vernemq_k8s: [
+            strategy: Elixir.Cluster.Strategy.Kubernetes,
+            config: [
+              mode: :hostname,
+              kubernetes_service_name: "astarte-vernemq",
+              kubernetes_node_basename: "VerneMQ",
+              kubernetes_ip_lookup_mode: :pods,
+              kubernetes_selector: vernemq_clustering_kubernetes_selector!(),
+              kubernetes_namespace: clustering_kubernetes_namespace!(),
+              polling_interval: 10_000
+            ]
           ]
         ]
 
@@ -526,6 +546,14 @@ defmodule Astarte.DataUpdaterPlant.Config do
               query: "astarte-data-updater-plant",
               node_basename: "astarte_data_updater_plant"
             ]
+          ],
+          vernemq: [
+            strategy: Elixir.Cluster.Strategy.DNSPoll,
+            config: [
+              polling_interval: 5_000,
+              query: "vernemq",
+              node_basename: "VerneMQ"
+            ]
           ]
         ]
     end