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/README.md

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+# Role `msmtp`
+
+A role for setting up msmtp for mail sending.
+
+The role only supports mail servers supporting either STARTTLS or SMTPS.
+
+## Supported Distributions
+
+Should work on Debian-based distributions.
+
+## Required Arguments
+
+- `msmtp__smtp_host`: The SMTP host to use.
+- `msmtp__smtp_port`: The SMTP port to use.
+- `msmtp__smtp_tls_method`: The SMTP TLS method to use.  
+  Possible choices:
+  - `starttls`: Use STARTTLS to connect to the server.
+  - `smtps`: Use SMTPS to connect to the server.
+- `msmtp__smtp_user`: The SMTP user to use for authentication.
+- `msmtp__smtp_password`: The SMTP password to use for authentication.
+- `msmtp__smtp_from`: The SMTP from address to use when sending mails.

roles/msmtp/meta/argument_specs.yaml

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+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

roles/msmtp/tasks/main.yaml

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+- name: ensure msmtp is installed
+  ansible.builtin.apt:
+    name: msmtp
+    state: present
+  become: true
+
+- name: ensure msmtp config for root user
+  ansible.builtin.template:
+    src: msmtprc.j2
+    dest: /root/.msmtprc
+    owner: root
+    group: root
+    mode: "0600"
+  become: true

roles/msmtp/templates/msmtprc.j2

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+# ansible-managed
+
+# defaults
+defaults
+auth on
+tls on
+
+# ansible-managed-account
+account ansible-managed-account
+host {{ msmtp__smtp_host }}
+port {{ msmtp__smtp_port }}
+tls_starttls {% if msmtp__smtp_tls_method == "starttls" %}on{% else %}off{% endif +%}
+user {{ msmtp__smtp_user }}
+password {{ msmtp__smtp_password }}
+from {{ msmtp__smtp_from }}
+
+account default: ansible-managed-account