Feature/add mailer service

Adds a mailer service to GSO and some config required in oss-params which I will update after this MR, including GAP deployment

gso/services/mailer.py

+"""The mailer service sends notification emails, as part of workflows that require interaction with external parties."""
+
+import smtplib
+from email.message import EmailMessage
+from ssl import create_default_context
+
+from gso.settings import load_oss_params
+
+
+def send_mail(recipient: str, subject: str, body: str) -> None:
+    """Send an email message to the given address.
+
+    Only supports STARTTLS, not SSL.
+
+    :param str recipient: The destination address.
+    :param str subject: The email subject.
+    :param str body: The contents of the email message.
+    """
+    email_params = load_oss_params().EMAIL
+    msg = EmailMessage()
+    msg["From"] = email_params.from_address
+    msg["To"] = recipient
+    msg["Subject"] = subject
+    msg.set_content(body)
+
+    with smtplib.SMTP(email_params.smtp_host, email_params.smtp_port) as s:
+        if email_params.starttls_enabled:
+            tls_context = create_default_context()
+            s.starttls(context=tls_context)
+        if email_params.smtp_username and email_params.smtp_password:
+            s.login(email_params.smtp_username, email_params.smtp_password)
+        s.send_message(msg)