Following commits make it possible to set a specific
partitioner for a table. We want to persist that information
and include it into schema digest. For that a new column
in scylla_tables is needed. This commit adds such column.
We add the new column to scylla_tables because it's a Scylla
specific extension.
Signed-off-by: Piotr Jastrzebski <piotr@scylladb.com>