Resolve #3588546: Add content entity cache tags to auto-saves/pending response cacheability

Feliksas Mazeikisrequested to merge
issue/canvas-3588546:3588546-add-content-entity into 1.x

Adds enforceIsNew(FALSE) call when creating a new entity in the AutoSaveManager::getAllAutoSaveList(). Updates existing tests accordingly. Adds additional tests to prove that endpoint cache clears on entity cache tag invalidation.

Closes #3588546

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