Initial ``rspamd`` Configuration

[Mischback]

Introduce rspamd into the setup.

For this step, simply make rspamd work.

• [ ] include rspamd from its own repositories
• [ ] adjust the default configuration slightly
  • [ ] make as much of its modules use redis as possible
  • [ ] deactivate real-time block lists (RBL)

Reference

• https://workaround.org/bullseye/filtering-out-spam-with-rspamd/
• https://www.rspamd.com/downloads.html#system2
• https://www.rspamd.com/doc/quickstart.html
• https://github.com/rspamd/rspamd