Create ChangeTagsUpdate hook

Create ChangeTagsUpdate hook

This creates a hook triggered when updating change tags, so that
extensions can take actions when tags are updated.
When tagging accompanies the action, the recent change is passed
while when tagging is subsequent to the action, the user who
performs the tagging is passed.

Bug: T118698
Change-Id: Ifb0cdc43252c4185e4f216d23b8a21fb31595a37