Do not allocate anything in subtransaction memory context (#12176)

## Problem

See https://github.com/neondatabase/neon/issues/12173

## Summary of changes

Allocate table in TopTransactionMemoryContext

---------

Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>

test_runner/sql_regress/sql/neon-subxacts.sql

@@ -0,0 +1,21 @@
+DO $$
+DECLARE
+i numeric;
+BEGIN
+  create role somebody;
+  FOR i IN 1..1000000 LOOP
+    BEGIN
+	  IF i % 1000 = 0 THEN
+	    alter role somebody password 'welcome';
+	  ELSE
+        PERFORM 1;
+	  END IF;
+    EXCEPTION WHEN OTHERS THEN
+      RAISE WARNING 'error';
+    END;
+    IF I = 1000000 THEN
+      PERFORM pg_log_backend_memory_contexts(pg_backend_pid());
+    END IF;
+  END LOOP;
+END;
+$$;