Issue #3605435 by hydra: Add canonical site filter support for Views

Port of erdfisch MR 195. Adds QueryableCanonicalSiteInterface (extends QueryableSiteInterface with canonicalQueryAlter()). The Sites views filter gains an optional 'Canonical only' exposed checkbox (with its own expose/visibility options and config schema), and SitesContentSql::doQueryAlter() calls canonicalQueryAlter() instead of queryAlter() for sites implementing the interface when that checkbox is active.

Prerequisite for sites_group #3595527 (GroupSitesBase implementing the interface).

Closes #3605435