core: introduce event_on_change function

Since we are going to move iproto override mechanism to the trigger
registry, we need to allow to set core triggers to on change event
because we need to notify iproto threads when a handler is overridden.
Let's add a new method that accepts a core trigger and sets it to an
internal trigger list.

Needed for #8138

NO_DOC=internal
NO_CHANGELOG=internal