Take into account default_tablespace during MERGE/SPLIT PARTITION(S)

Take into account default_tablespace during MERGE/SPLIT PARTITION(S)

createPartitionTable() passed the partitioned parent's reltablespace straight
to heap_create_with_catalog(), bypassing the default_tablespace GUC fallback
that DefineRelation() applies for CREATE TABLE ... PARTITION OF.  When the
parent had no explicit tablespace (reltablespace = 0), the new partition
unconditionally landed in the database default, even if default_tablespace
was set to something else; merging or splitting a set of partitions that all
lived in a non-default tablespace produced a new partition in the database
default.

Mirror DefineRelation()'s logic: take parent's reltablespace if set,
otherwise check GetDefaultTablespace() (which reads default_tablespace
and normalises pg_default / MyDatabaseTableSpace to InvalidOid).  Also
add the CREATE ACL check on the resolved tablespace and the pg_global
rejection, matching DefineRelation()'s behavior.

Update the documentation for MERGE/SPLIT PARTITION to spell out the
tablespace-selection rule explicitly.

Reported-by: Justin Pryzby <pryzby@telsasoft.com>
Reviewed-by: Pavel Borisov <pashkin.elfe@gmail.com>
Discussion: https://postgr.es/m/ajQTklv8QArzTp3h%40pryzbyj2023