Issue #3595527: Implement QueryableCanonicalSiteInterface on GroupSitesBase

Pascal Crottrequested to merge
issue/sites_group-3595527:3595527-queryablecanonicalsiteinterface-canonical-only-views into 1.x

Adds canonicalQueryAlter(), so sites_group sites can support the "Canonical only" checkbox exposed by Drupal\sites\Plugin\views\filter\Sites for sites implementing QueryableCanonicalSiteInterface.

Closes #3595527

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