In ErrorIfShardPlacementsNotColocated(), while checking if shards are colocated,
error out if matching shard intervals have different number of shard placements.
Added a new UDF, mark_tables_colocated(), to colocate tables with the same
configuration (shard count, shard replication count and distribution column type).
With this change, we now push down foreign key constraints created during CREATE TABLE
statements. We also start to send foreign constraints during shard move along with
other DDL statements