[Query engine] Handle cross-shard check in schema sharding with no sharded tables

[levkk]

Description

SELECT queries are routed to a random shard if no search_path is set and they are not fully qualified if using schema-based sharding. When cross_shard_disabled = true, this bypasses the check and allows cross-shard queries through. The results returned are still accurate if search_path is set, or it will return an occasional error when RR hits a shard that doesn't have that table.