pgsql: Allow private state in certain planner data structures.

Allow private state in certain planner data structures.

Extension that make extensive use of planner hooks may want to
coordinate their efforts, for example to avoid duplicate computation,
but that's currently difficult because there's no really good way to
pass data between different hooks.

To make that easier, allow for storage of extension-managed private
state in PlannerGlobal, PlannerInfo, and RelOptInfo, along very
similar lines to what we have permitted for ExplainState since commit
c65bc2e1d14a2d4daed7c1921ac518f2c5ac3d17.

Reviewed-by: Andrei Lepikhov <lepihov@gmail.com>
Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: http://postgr.es/m/CA+TgmoYWKHU2hKr62Toyzh-kTDEnMDeLw7gkOOnjL-TnOUq0kQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0132dddab33a6eae2d3d63eda9f053e745fedb06

Modified Files
--------------
src/backend/optimizer/util/Makefile     |   1 +
src/backend/optimizer/util/extendplan.c | 183 ++++++++++++++++++++++++++++++++
src/backend/optimizer/util/meson.build  |   1 +
src/include/nodes/pathnodes.h           |  12 +++
src/include/optimizer/extendplan.h      |  72 +++++++++++++
5 files changed, 269 insertions(+)