pgsql: Take into account default_tablespace during MERGE/SPLIT PARTITIO

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(at)telsasoft(dot)com>
Reviewed-by: Pavel Borisov <pashkin(dot)elfe(at)gmail(dot)com>
Discussion: https://postgr.es/m/ajQTklv8QArzTp3h%40pryzbyj2023

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/cdae794af31b3e9cfc323fc654292d86fa746f77

Modified Files
--------------
doc/src/sgml/ref/alter_table.sgml | 18 +++++--
src/backend/commands/tablecmds.c | 31 +++++++++++-
src/test/regress/expected/partition_merge.out | 53 +++++++++++++++++++++
src/test/regress/expected/partition_split.out | 68 +++++++++++++++++++++++++++
src/test/regress/sql/partition_merge.sql | 40 ++++++++++++++++
src/test/regress/sql/partition_split.sql | 52 ++++++++++++++++++++
6 files changed, 257 insertions(+), 5 deletions(-)