pgsql: injection_points: Add injection_points_list()

injection_points: Add injection_points_list()

This function can be used to retrieve the information about all the
injection points attached to a cluster, providing coverage for
InjectionPointList() introduced in 7b2eb72b1b8c.

The original proposal turned around a system function, but that would
not be backpatchable to stable branches. It was also a bit weird to
have a system function that fails depending on if the build allows
injection points or not.

Reviewed-by: Aleksander Alekseev <aleksander(at)timescale(dot)com>
Reviewed-by: Rahila Syed <rahilasyed90(at)gmail(dot)com>
Discussion: https://postgr.es/m/Z_xYkA21KyLEHvWR@paquier.xyz

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/4eca711bc991954613261b7a314b1e8f5963815c

Modified Files
--------------
.../injection_points/expected/injection_points.out | 16 +++++++++
.../injection_points/injection_points--1.0.sql | 12 +++++++
.../modules/injection_points/injection_points.c | 39 ++++++++++++++++++++++
.../injection_points/sql/injection_points.sql | 7 ++++
4 files changed, 74 insertions(+)