The wiki has documented "21. Set notification for customscript" since
2022 but the implementation (#4193) was never merged, so following the
wiki failed with "Cannot find the hook file". Same interface as
documented: the script gets subject, content and status code as three
arguments. Unlike #4193, the target script is invoked directly instead
of through eval -- the subject/content contain domain names and CA
messages, and eval would allow command injection through them.
https://github.com/acmesh-official/acme.sh/issues/6377
