Fix realtime_changes import endpoints ETag behavior

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According to !83849 (comment 896168862) not every realtime_changes_import_${import_type} endpoint is registered within ETag router. This is definitely bugperformance for us

We need to address this is in one of the following ways:

• confirm that this is weird, yet expected
• add all realtime_changes import endpoints to ETag router
• refactor to use one realtime_changes endpoint (within new import/history_controller which makes sense, since realtime_changes are part of the history) instead of having one endpoint per importer (all endpoints are ATM using same logic, specified in import/base_controller.rb
• or even move all logic to GraphQL Subscriptions, which is doable and acceptable path for now