Add PostHog consent integration and extract integration plugin system

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>

pkg/server/api/console/v1/graphql/cookie_banner.graphql

@@ -124,6 +124,7 @@ type CookieCategory implements Node {
     kind: CookieCategoryKind!
     rank: Int!
     gcmConsentTypes: [String!]!
+    posthogConsent: Boolean!
 
     cookies(
         first: Int
@@ -310,6 +311,7 @@ input UpdateCookieCategoryInput {
     slug: String
     description: String
     gcmConsentTypes: [String!]
+    posthogConsent: Boolean
 }
 
 input DeleteCookieCategoryInput {