refactor: dynamic SMTP for self host (#138)

Co-authored-by: KM Koushik <koushikmohan1996@gmail.com>

.env.selfhost.example

@@ -12,6 +12,10 @@ DATABASE_URL="postgresql://postgres:postgres@postgres:5432/unsend"
 NEXTAUTH_URL="http://localhost:3000"
 NEXTAUTH_SECRET=
 
+#SMTP
+NEXT_PUBLIC_SMTP_HOST=smtp.mailtrap.io  # Example SMTP host
+NEXT_PUBLIC_SMTP_USER= "unsend" # Example SMTP user
+
 ## Auth providers any one is required
 # Github login - required
 GITHUB_ID="<your-github-client-id>"