importccl: the userfile table name generation algorithm breaks with special usernames, and confused client-side and server-side usernames

This code:

```
 qualifiedTableName = DefaultQualifiedNamePrefix + user
```

(`external_storage.go`)

breaks when `user` starts with digits, or when it contains periods. Username consisting of only digits, or containing periods, have been allowed since crdb v19.2.

The code should instead quote the username properly.

