Because of https://github.com/liferay/liferay-portal/blob/master/modules/apps/captcha/captcha-api/src/main/java/com/liferay/captcha/configuration/CaptchaConfiguration.java#L66, it is impossible to add any additional captcha engine without modifying core Liferay classes.
We should not restrict to this finite list of two values but allow any FQCN.
In addition to that, in order to provide an initial list of existing implementations, we should rely on https://help.liferay.com/hc/en-us/articles/360032990191-Dynamically-Populating-Select-List-Fields-in-the-Configuration-UI which would allow a developer to override the list.
- As a Developer, I want to be able to add the option to choose additional captcha engines to the default ootb supported list of captcha engines without modifying core classes.
- As a Developer, I want to be able to add additional captcha engines to the list fo the options in order to give the option to choose them to Administrators.