Automated UDT support

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>

doc_test.go

@@ -0,0 +1,27 @@
+package gocqlx_test
+
+import (
+	"github.com/scylladb/gocqlx"
+)
+
+func ExampleUDT() {
+	// Just add gocqlx.UDT to a type, no need to implement marshalling functions
+	type FullName struct {
+		gocqlx.UDT
+		FirstName string
+		LastName  string
+	}
+}
+
+func ExampleUDT_wraper() {
+	type FullName struct {
+		FirstName string
+		LastName  string
+	}
+
+	// Create new UDT wrapper type
+	type FullNameUDT struct {
+		gocqlx.UDT
+		*FullName
+	}
+}