python infra to measure total-space-efficiency & point-in-time space efficiency

[problame]

Meeting notes / ideas:

• point-in-time efficiency: Use layer map dump / keyspace dump (not actually measure it by access)
• total space efficiency:
  • (logical size + wal in PITR window) = synthetic size, can just use that
  • sum(all layer files in index_part.json) => just that

Demo test case to adapt / apply the Python helpers to:

• test_gc_feedback

[problame]

This week:

• extend / follow-up on the pagectl PR test(pageserver): quantify compaction outcome  #7867 so that it runs the estimations against test_gc_feedback