use_toja can not be applied to a named query block & leading hint can not be applied to join children

## Enhancement

* use_toja should be able to be applied to a named query block
* when we build a join, we will set the preferred join hint, we could see when join child are already a join with merged schema(different table name inside), it couldn't pass the extract alias check
```
func (p *LogicalJoin) setPreferredJoinTypeAndOrder(hintInfo *tableHintInfo) {
if hintInfo == nil {
return
}

lhsAlias := extractTableAlias(p.children[0], p.blockOffset)
rhsAlias := extractTableAlias(p.children[1], p.blockOffset)

func extractTableAlias(p Plan, parentOffset int) *hintTableInfo {
if len(p.OutputNames()) > 0 && p.OutputNames()[0].TblName.L != "" {
firstName := p.OutputNames()[0]
for _, name := range p.OutputNames() {
if name.TblName.L != firstName.TblName.L ||
(name.DBName.L != "" && firstName.DBName.L != "" && name.DBName.L != firstName.DBName.L) { // DBName can be nil, see #46160
return nil
}
}
```
as we see from the extractTableAlias, when the output name contains two different table, it will abort the extracting logic.
that's why case 7458 couldn't succeed.