[ZendTo] Adding Support for Cloudflare's Turnstyle for Captcha/Human Verification

[Ricky Boone]

My apologies if this has already been covered, however I couldn't find
anything other than a single email in the list archive that didn't
appear to be addressed.

The users of the instance of ZendTo that I maintain sometimes have
issues with reCAPTCHA.  In almost all cases, it's either user-oriented
or an issue with their browser causing them to never get past it, with
the very rare case of an issue on Google's end.  I'd still like some
reasonable mechanism to perform human verification to reduce the risk
posed without it, however it looks like the only options available are
either Google's standard or invisible reCAPTCHA v2 services, with the
"invisible" option having noted issues since 2018 (at least in the
config comments).

So I don't start going down a path that is already underway, are there
any plans to implement support for other CAPTCHA services/libraries,
such as Cloudflare's Turnstyle or hCaptcha?  If not, are there any
concerns with me taking a stab at it and providing what I come up
with?  I realize that ZendTo is not being tracked on a public source
code repo like Github or GitLab (at least officially, as far as I can
tell), but I didn't want to do something that couldn't be contributed
back in some way.