Make gocqlx part of the package

[dkropachev]

I observed some unsatisfied interest in having gocqlx for https://github.com/apache/cassandra-gocql-driver:
scylladb/gocqlx#330
scylladb/gocqlx#334

Could you please take a look to onboard gocqlx either as separate repo, or as part of https://github.com/apache/cassandra-gocql-driver.