Backpatching injection point core facilities to REL_17_STABLE

Hi all,

As mentioned during my talk about injection points at the last
pgconf.dev, I think that we should be more aggressive with the
backpatching of the core APIs of this facility, to ease the porting of
tests across more branches and the maintenance of this stuff itself,
so as it evolves in a consistent way across all stable branches.

Backporting that down to v16 would require more work, like in the area
of custom wait events for the injection point category for isolation
tests. The story is different for v17, where most of the basics are
in place. Please see the attached, that adds the following parts to
REL_17_STABLE:
- Cached points and loading, for critical sections.
- IS_INJECTION_POINT_ATTACHED(), for stack manipulations.
- Runtime arguments.

Thoughts or comments?
--
Michael