qb: Added shortcuts to Queryx

It allows to write shorter and more straightforward code.
Instead writing:

```
session.Query(qb.Select("cluster").Columns("id").ToCql())
```

you can write:

```
qb.Select("cluster").Columns("id").Query(session)
```

qb/select.go

@@ -10,6 +10,8 @@ package qb
 import (
 	"bytes"
 	"fmt"
+
+	"github.com/scylladb/gocqlx/v2"
 )
 
 // Order specifies sorting order.
@@ -118,6 +120,11 @@ func (b *SelectBuilder) ToCql() (stmt string, names []string) {
 	return
 }
 
+// Query returns query built on top of current SelectBuilder state.
+func (b *SelectBuilder) Query(session gocqlx.Session) *gocqlx.Queryx {
+	return session.Query(b.ToCql())
+}
+
 // From sets the table to be selected from.
 func (b *SelectBuilder) From(table string) *SelectBuilder {
 	b.table = table