Add PostHog opt-in/opt-out consent support mirroring the existing Google
Consent Mode integration: database column, GraphQL field, console UI
toggle, and client-side posthog-js calls.
Extract both GCM and PostHog logic from CookieBannerClient into a
ConsentIntegration plugin interface so future integrations can be added
without modifying the client core.
Signed-off-by: Émile Ré <emile@getprobo.com>
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