mCaptcha support

[SadmL]

Description:

Synapse only supports Google Captcha as says in configuration documentation
https://element-hq.github.io/synapse/latest/usage/configuration/config_documentation.html?highlight=captcha#captcha

Not everybody happy to use Google stuff for protection, it would be nice have an open source alternative such as mCaptcha for example.
https://mcaptcha.org/

Forgejo (and Gitea) already supports it.
https://forgejo.org/docs/latest/admin/config-cheat-sheet/#service-service
MCAPTCHA_ parameters

[sandhose]

This should be discussed in matrix-org/matrix-spec#295