Add pageserver parameter forced_image_creation_limit which can be used… (#4353)

This parameter can be use to restrict number of image layers generated
because of GC request (wanted image layers).
Been set to zero it completely eliminates creation of such image layers.
So it allows to avoid extra storage consumption after merging #3673

## Problem
PR #3673 forces generation of missed image layers. So i short term is
cause cause increase (in worst case up to two times) size of storage.
It was intended (by me) that GC period is comparable with PiTR interval.
But looks like it is not the case now - GC is performed much more
frequently. It may cause the problem with space exhaustion: GC forces
new image creation while large PiTR still prevent GC from collecting old
layers.

## Summary of changes

Add new pageserver parameter` forced_image_creation_limit` which
restrict number of created image layers which are requested by GC.

## Checklist before requesting a review

- [ ] I have performed a self-review of my code.
- [ ] If it is a core feature, I have added thorough tests.
- [ ] Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?
- [ ] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.

## Checklist before merging

- [ ] Do not forget to reformat commit message to not include the above
checklist

test_runner/regress/test_attach_tenant_config.py

@@ -158,6 +158,7 @@ def test_fully_custom_config(positive_env: NeonEnv):
             "threshold": "23h",
         },
         "evictions_low_residence_duration_metric_threshold": "2days",
+        "gc_feedback": True,
         "gc_horizon": 23 * (1024 * 1024),
         "gc_period": "2h 13m",
         "image_creation_threshold": 7,