Add an local captcha module

[fMichaleczek]

It could be nice to provide a captcha module
without any external server dependencies on closed-source.

It seems that FriendlyCaptcha is the only one open source projet which can be hosted by yourself.
https://docs.friendlycaptcha.com/

Do you know others solutions ?
Do you think it could add a great value to Orchard ?

[Skrypt]

Please look at this PR which allows extending Captcha :
#5932
I did not have time to review it properly but it looked fine on first look.