Opt-In Feature: Collect IP addresses of spammers for auto-blacklist

[stklcode]

Feature suggested in the WP support forums (https://wordpress.org/support/topic/blacklisten-bitte-zur-auswahl-stellen/#post-10381313 - German).

IP addresses of blocked visitors (by any metric except IP-blacklist itself) should be collected in a local blacklist (similar to the approach of AntispamBee for comments) and automatically excluded from tracking.

This feature has to be an opt-in and store addresses anonymized (at least a salted hash) to not break GDPR compliance.