feat(pageserver): gc-compaction result verification (#11515)

## Problem

Part of #9114 

There was a debug-mode verification mode that verifies at every
retain_lsn. However, the code was tangled within the actual history
generation itself and it's hard to reason about correctness. This patch
adds a separate post-verification of the gc-compaction result that redos
logs at every retain_lsn and every record above the GC horizon. This
ensures that all key history we produce with gc-compaction is readable,
and if there're read errors after gc-compaction, it can only be
read-path errors instead of gc-compaction bugs.

## Summary of changes

* Add gc_compaction_verification flag, default to true.
* Implement a post-verification process.

---------

Signed-off-by: Alex Chi Z <chi@neon.tech>

test_runner/regress/test_attach_tenant_config.py

@@ -187,6 +187,7 @@ def test_fully_custom_config(positive_env: NeonEnv):
         },
         "rel_size_v2_enabled": False,  # test suite enables it by default as of https://github.com/neondatabase/neon/issues/11081, so, custom config means disabling it
         "gc_compaction_enabled": True,
+        "gc_compaction_verification": False,
         "gc_compaction_initial_threshold_kb": 1024000,
         "gc_compaction_ratio_percent": 200,
         "image_creation_preempt_threshold": 5,