Add AuthenSASL to perl deps; improve email docs

The addition of AuthenSASL seems to be necessary to properly
authenticate against an SMTP server. Without this I got errors
such as

    error with Hydra::Plugin::EmailNotification=HASH(0x6ad0128)->buildFinished: SMTP auth requires MIME::Base64 and Authen::SASL

doc/manual/src/projects.md

@@ -458,6 +458,45 @@ Hydra can send email notifications when the status of a build changes.
 This provides immediate feedback to maintainers or committers when a
 change causes build failures.
 
+The feature can be turned on by adding the following line to `hydra.conf`
+
+``` conf
+email_notification = 1
+```
+
+By default, Hydra only sends email notifications if a previously successful
+build starts to fail. In order to force Hydra to send an email for each build
+(including e.g. successful or cancelled ones), the environment variable
+`HYDRA_FORCE_SEND_MAIL` can be declared:
+
+``` nix
+services.hydra-dev.extraEnv.HYDRA_FORCE_SEND_MAIL = "1";
+```
+
+SASL Authentication for the email address that's used to send notifications
+can be configured like this:
+
+``` conf
+EMAIL_SENDER_TRANSPORT_sasl_username=hydra@example.org
+EMAIL_SENDER_TRANSPORT_sasl_password=verysecret
+EMAIL_SENDER_TRANSPORT_port=587
+EMAIL_SENDER_TRANSPORT_ssl=starttls
+```
+
+Further information about these environment variables can be found at the
+[MetaCPAN documentation of `Email::Sender::Manual::QuickStart`](https://metacpan.org/pod/Email::Sender::Manual::QuickStart#specifying-transport-in-the-environment).
+
+It's recommended to not put this in `services.hydra-dev.extraEnv` as this would
+leak the secrets into the Nix store. Instead, it should be written into an
+environment file and configured like this:
+
+``` nix
+{ systemd.services.hydra-notify = {
+    serviceConfig.EnvironmentFile = "/etc/secrets/hydra-mail-cfg";
+  };
+}
+```
+
 The simplest approach to enable Email Notifications is to use the ssmtp
 package, which simply hands off the emails to another SMTP server. For
 details on how to configure ssmtp, see the documentation for the

flake.nix

@@ -290,6 +290,7 @@
             name = "hydra-perl-deps";
             paths = with perlPackages; lib.closePropagation
               [ ModulePluggable
+                AuthenSASL
                 CatalystActionREST
                 CatalystAuthenticationStoreDBIxClass
                 CatalystAuthenticationStoreLDAP