sql: add a default AOST option to CREATE STATISTICS

Using CREATE STATISTICS without an AOST option results in a regular scan
which could contend with concurrent transactions. Implements a default
AOST of -1us to the CREATE STATISTICS command to avoid this.

Fixes: cockroachdb#72719

Release note (sql change): using the CREATE STATISTICS command without
the AS OF SYSTEM TIME option could contend with concurrent transactions
and cost performance. Running CREATE STATISTICS without specifying AS OF
SYSTEM TIME now uses a default of -1us.

pkg/sql/logictest/testdata/logic_test/distsql_event_log

@@ -31,7 +31,7 @@ FROM system.eventlog
 WHERE "eventType" = 'create_statistics' AND ("info"::JSONB ->> 'DescriptorID')::INT = 106
 ORDER BY "timestamp", info
 ----
-1  {"EventType": "create_statistics", "Statement": "CREATE STATISTICS s1 ON id FROM test.public.a", "TableName": "test.public.a", "Tag": "CREATE STATISTICS", "User": "root"}
+1  {"EventType": "create_statistics", "Statement": "CREATE STATISTICS s1 ON id FROM test.public.a WITH OPTIONS AS OF SYSTEM TIME '-1us'", "TableName": "test.public.a", "Tag": "CREATE STATISTICS", "User": "root"}
 
 statement ok
 DROP TABLE a