Config for special mobs to spawn only after the End

Simple suggest for a configurable option where these mobs will only able to spawn after The End was accessed.

A better setting would be that these mobs will only spawn around players who have visited The End.

The issue names no files, tests, or entry points, so first clarify which mobs and configuration mechanism are in scope. Define whether spawning is gated globally after The End is accessed or per player, then verify that the chosen behavior is configurable and works as described.

Written by the indexing model from the issue text.