Self-hosted Captcha Support via mCaptcha

[0xdade]

Is your feature request related to a problem? Please describe.
One of the primary reasons I want to use listmonk is because I am self hosting as much as I can and making as much an effort as I can to reduce tracking of users who visit my sites. I would love to use a captcha to deter abuse of my subscribe forms, but I don't want to use a hosted solution like hcaptcha.

Describe the solution you'd like
mCaptcha is a self hosted captcha solution that is API-compatible with hCaptcha (at least according to the homepage). I believe that to enable support for mCaptcha servers, the only change needed for listmonk would be an additional configuration in the captcha settings, to optionally set a URL. This can default to hcaptcha if not set, but optionally be overridden to use a self-hosted mcaptcha instead.