Add a single ops feature flag to disable all ddl across the gitlab database

During postgres upgrades, DBREs need to turn off a large quantity of feature flags, for example (not an exhaustive list):

• async_foreign_key
• async_index
• partition_manager_sync_partitions

For future upgrades and other database maintenance procedures, this list could grow, and it becomes unwieldy to remember all the flags.

Deliverables

• Identify and enumerate places we perform DDL within the application.
• • partition_manager_sync_partitions
• • execute_batched_migrations_on_schedule
• • execute_background_migrations
• • database_reindexing
• • database_async_index_operations
• • database_async_foreign_key_validation
• • database_async_index_creation
• Introduce a single flag that, when disabled, disables all ddl across the database.
• • !130554 (merged)
• Existing feature flags still work as expected, disabling individual features
• Document the flag... somewhere?
• • gitlab-com/runbooks!6304 (merged)