Add keyspace parameter to CassandraSchema and CassandraTable

In our database solution, we construct `CassandraSchema` instances ourselves (bypassing `CassandraSchemaFactory`) as users may configure Cassandra datasources without specifying a default keyspace.

Currently, it is not possible to construct a `CassandraSchema` instance if the keyspace is not set in the CQL session.

Furthermore, queries generated by `CassandraTable` do not include a keyspace and will fail if the CQL session does not have a keyspace set.