pgsql: Support loading of injection points

Support loading of injection points

This can be used to load an injection point and prewarm the
backend-level cache before running it, to avoid issues if the point
cannot be loaded due to restrictions in the code path where it would be
run, like a critical section where no memory allocation can happen
(load_external_function() can do allocations when expanding a library
name).

Tests can use a macro called INJECTION_POINT_LOAD() to load an injection
point.  The test module injection_points gains some tests, and a SQL
function able to load an injection point.

Based on a request from Andrey Borodin, who has implemented a test for
multixacts requiring this facility.

Reviewed-by: Andrey Borodin
Discussion: https://postgr.es/m/ZkrBE1e2q2wGvsoN@paquier.xyz

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/4b211003ecc2946dc0052b650141ea4e8f35254c

Modified Files
--------------
doc/src/sgml/xfunc.sgml                            |  14 +++
src/backend/utils/misc/injection_point.c           | 121 +++++++++++++++------
src/include/utils/injection_point.h                |   3 +
.../injection_points/expected/injection_points.out |  32 ++++++
.../injection_points/injection_points--1.0.sql     |  10 ++
.../modules/injection_points/injection_points.c    |  17 +++
.../injection_points/sql/injection_points.sql      |   7 ++
7 files changed, 168 insertions(+), 36 deletions(-)