Statementsummary v2 support for TIDB_STATEMENTS_STATS

## Enhancement

On cloud, stmtv2 is enabled to collect stmt log, i.e. `tidb_stmt_summary_enable_persistent=true`. But if user access workload repository tables, errors will be thrown. Since culmulated stats are not adapted to v2, there is also no column factory.

```
mysql> select DIGEST_TEXT,MEM from information_schema.TIDB_STATEMENTS_STATS;
ERROR 1105 (HY000): should never happen, should register new column MEM into columnValueFactoryMap
mysql>
```

Start by tracing the TIDB_STATEMENTS_STATS implementation and its columnValueFactoryMap, then reproduce the stated query with stmtv2 enabled. Done means workload repository table access no longer raises the missing-column-factory error and the requested statistics columns are supported.

Written by the indexing model from the issue text.