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@timescale.com>
Reviewed-by: Rahila Syed <rahilasyed90@gmail.com>
Discussion: https://postgr.es/m/Z_xYkA21KyLEHvWR@paquier.xyz