The example post-data hook was missing a quote around a sub-shell
execution.
This is harmless because the content itself is admin-provided and not
related to user input, but this commit fixes the quote for defense in
depth and consistency.
Rspamd (https://rspamd.com/) is a popular open-source spam filtering
system.
This patch adds integration with it in the example hook, which uses the
rspamc client to get a veredict, similar to what it does for
Spamassassin.
This patch adds DKIM signing using https://github.com/driusan/dkim tools
to the example hook.
It also adds an optional integration test to exercise signing and
verification, and corresponding documentation.
