msmtp(role): introduce msmtp role

Introduce msmtp role for setting up msmtp for mail sending.
Also add accompanying host group and playbook play.

roles/msmtp/meta/argument_specs.yaml

@@ -0,0 +1,24 @@
+argument_specs:
+  main:
+    options:
+      msmtp__smtp_host:
+        type: str
+        required: true
+      msmtp__smtp_port:
+        type: int
+        required: true
+      msmtp__smtp_tls_method:
+        type: str
+        required: true
+        choices:
+          - "starttls"
+          - "smtps"
+      msmtp__smtp_user:
+        type: str
+        required: true
+      msmtp__smtp_password:
+        type: str
+        required: true
+      msmtp__smtp_from:
+        type: str
+        required: true