Introduce ParserCacheSaveComplete hook.

Introduce ParserCacheSaveComplete hook.

Rationale: give extensions a way to track which "renderings"
of a page exist in the cache. This is particularly relevant
for multi-lingual wikis that splpit the parser cache by user
language on some pages. In that case, hooking into
ParserAfterParse or LinksUpdateComplete is insufficient to
track all language specific renderings.

Bug: T99511
Change-Id: Iebf526098ca837a7df637c650097119495000c81