Add send_only_mailserver role and deploy Send-Only-Mailserver with it

Co-authored-by: yuri <yuri@nekover.se>

playbooks/roles/send_only_mail_server/README.md

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+# Role `send_only_mail_server`
+
+Makes sure a send-only mail server is deployed using OpenSMTPD and Rspamd for DKIM signing.
+
+Make sure to manually set a DMARC record and MX record for the mail domains.
+
+## Supported Distributions
+
+The following distributions are supported:
+
+- Debian 11
+
+## Required Arguments
+
+For the required arguments look at the [`argument_specs.yaml`](./meta/argument_specs.yml)
+
+Also make sure to set the following for the `cert` role dependency:
+
+- `cert__acme_account_email`
+
+## Updates
+
+This role doesn't handle updates.
+However it uses the system package manager for installing all the packages, so when you're making sure the system packages are up-to-date, you're handling updates for the packages installed by this role as well.
+
+## `hosts`
+
+The `hosts` for this role need to be the machines on which you want to deploy a mail server.
+
+## Links & Resources
+
+- <https://www.opensmtpd.org/>
+- <https://www.opensmtpd.org/manual.html>
+- <https://www.rspamd.com/>
+- <https://poolp.org/posts/2019-09-14/setting-up-a-mail-server-with-opensmtpd-dovecot-and-rspamd/>