We can't return an error in case a field is added to the UDT,
otherwise existing code would break by simply altering the UDT in the
database. For extra fields at the end of the UDT put nulls to be in
line with gocql, but also python-driver and java-driver.
In gocql it was fixed in d2ed1bb74f
This patch adds the power of GocqlX to UDTs.
Now you can make a struct be UDT compatible by adding a single line.
```
type FullName struct {
gocqlx.UDT
FirstName string
LastName string
}
```
Signed-off-by: Michał Matczuk <michal@scylladb.com>