Support multiple columns in column copy triggers

What does this MR do?

Relates to #326569 (closed)

In migration helpers we use "rename" triggers for certain migrations, whenever we need to ensure a destination column keeps the same value as a source column. We need to extend this functionality to support multiple source and destination columns, for migrations where we need to update more than one column at a time. At the same time, we need to maintain backwards compatibility with existing behavior to not break any existing migrations.

That means the install_rename_triggers and related functions should:

1. Create triggers with the same names, same behavior, and same syntax (to prevent diff in structure.sql)
2. Retain compatibility with their existing method signature, while also supporting arrays of column names

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • [-] I have included a changelog entry.
  • [-] I have not included a changelog entry because _____.
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

lib/gitlab/database/migration_helpers.rb

@@ -577,17 +577,7 @@ def undo_rename_column_concurrently(table, old, new)
       # old_column - The name of the old column.
       # new_column - The name of the new column.
       def install_rename_triggers(table, old_column, new_column)
-        trigger_name = rename_trigger_name(table, old_column, new_column)
-        quoted_table = quote_table_name(table)
-        quoted_old = quote_column_name(old_column)
-        quoted_new = quote_column_name(new_column)
-
-        install_rename_triggers_for_postgresql(
-          trigger_name,
-          quoted_table,
-          quoted_old,
-          quoted_new
-        )
+        install_rename_triggers_for_postgresql(nil, table, old_column, new_column)
       end
 
       # Changes the type of a column concurrently.
@@ -1055,42 +1045,17 @@ def backfill_conversion_of_integer_to_bigint(
 
       # Performs a concurrent column rename when using PostgreSQL.
       def install_rename_triggers_for_postgresql(trigger, table, old, new)
-        execute <<-EOF.strip_heredoc
-        CREATE OR REPLACE FUNCTION #{trigger}()
-        RETURNS trigger AS
-        $BODY$
-        BEGIN
-          NEW.#{new} := NEW.#{old};
-          RETURN NEW;
-        END;
-        $BODY$
-        LANGUAGE 'plpgsql'
-        VOLATILE
-        EOF
-
-        execute <<-EOF.strip_heredoc
-        DROP TRIGGER IF EXISTS #{trigger}
-        ON #{table}
-        EOF
-
-        execute <<-EOF.strip_heredoc
-        CREATE TRIGGER #{trigger}
-        BEFORE INSERT OR UPDATE
-        ON #{table}
-        FOR EACH ROW
-        EXECUTE FUNCTION #{trigger}()
-        EOF
+        Gitlab::Database::UnidirectionalCopyTrigger.on_table(table).create(old, new, trigger_name: trigger)
       end
 
       # Removes the triggers used for renaming a PostgreSQL column concurrently.
       def remove_rename_triggers_for_postgresql(table, trigger)
-        execute("DROP TRIGGER IF EXISTS #{trigger} ON #{table}")
-        execute("DROP FUNCTION IF EXISTS #{trigger}()")
+        Gitlab::Database::UnidirectionalCopyTrigger.on_table(table).drop(trigger)
       end
 
       # Returns the (base) name to use for triggers when renaming columns.
       def rename_trigger_name(table, old, new)
-        'trigger_' + Digest::SHA256.hexdigest("#{table}_#{old}_#{new}").first(12)
+        Gitlab::Database::UnidirectionalCopyTrigger.on_table(table).name(old, new)
       end
 
       # Returns an Array containing the indexes for the given column