DEV: Support nullable column property modification (#32978)

By default, rails makes timestamp columns (`created_at` and
`updated_at`) non-nullable, we also have some required core and plugins
columns we wouldn't necessarily want to enforce in the intermediate DB
schema. It'll be better to set the default values for these during
import instead of enforcing these at the converter level.

This change adds support for globally modifying a column’s `nullable`
state, defaulting all `created_at` columns to be `nullable` while
allowing for table level overrides.

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Co-authored-by: Gerhard Schlager <gerhard.schlager@discourse.org>

migrations/lib/database/intermediate_db/user.rb

@@ -48,7 +48,7 @@ module Migrations::Database::IntermediateDB
       approved: nil,
       approved_at: nil,
       approved_by_id: nil,
-      created_at:,
+      created_at: nil,
       date_of_birth: nil,
       first_seen_at: nil,
       flair_group_id: nil,