API: Add a 'known' property when missing-but-known titles are output

API: Add a 'known' property when missing-but-known titles are output

For example file pages from foreign repos, MediaWiki-namespace messages
that haven't been locally customized, and titles manipulated with the
'TitleIsAlwaysKnown' hook.

This allows clients to be able to display such titles as bluelinks
rather than redlinks.

This also has ApiPageSet populate LinkCache to make later checks of
->exists() and ->isKnown() more efficient.

Bug: T141963
Change-Id: Idbdfd2896c0ce9425ededd7cb4b60eda89ba7ef5