pgsql: Add pg_partition_tree to display information about partitions

Add pg_partition_tree to display information about partitions

This new function is useful to display a full tree of partitions with a
partitioned table given in output, and avoids the need of any complex
WITH RECURSIVE query when looking at partition trees which are
deep multiple levels.

It returns a set of records, one for each partition, containing the
partition's name, its immediate parent's name, a boolean value telling
if the relation is a leaf in the tree and an integer telling its level
in the partition tree with given table considered as root, beginning at
zero for the root, and incrementing by one each time the scan goes one
level down.

Author: Amit Langote
Reviewed-by: Jesper Pedersen, Michael Paquier, Robert Haas
Discussion: https://postgr.es/m/8d00e51a-9a51-ad02-d53e-ba6bf50b2e52@lab.ntt.co.jp

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/d5eec4eefde70414c9929b32c411cb4f0900a2a9

Modified Files
--------------
doc/src/sgml/func.sgml                       |  43 ++++++++
src/backend/utils/adt/Makefile               |   4 +-
src/backend/utils/adt/partitionfuncs.c       | 154 +++++++++++++++++++++++++++
src/include/catalog/catversion.h             |   2 +-
src/include/catalog/pg_proc.dat              |   9 ++
src/test/regress/expected/partition_info.out | 114 ++++++++++++++++++++
src/test/regress/parallel_schedule           |   2 +-
src/test/regress/serial_schedule             |   1 +
src/test/regress/sql/partition_info.sql      |  68 ++++++++++++
9 files changed, 393 insertions(+), 4 deletions(-)