storage: add blob-rewrite compaction cluster settings

[jbowens]

Add two new cluster settings for configuring blob-rewrite compactions.

Epic: CRDB-20379
Release note: None

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[annrpom]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @RaduBerinde)

---

-- commits line 7 at r1:
nit: should we document this new cluster setting in the release note

[RaduBerinde]

Reviewable status: complete! 1 of 0 LGTMs obtained (and 1 stale) (waiting on @annrpom and @jbowens)

---

pkg/storage/pebble.go line 384 at r2 (raw file):

		settings.DurationWithMinimum(0),
	)
	valueSeparationCompactionGarbageThreshold = settings.RegisterFloatSetting(

[nit] Consider making this an integer percent (it's more user-friendly).

---

pkg/storage/pebble.go line 387 at r2 (raw file):

		settings.SystemVisible,
		"storage.value_separation.compaction_garbage_threshold",
		"the max garbage threshold configures the maximum fraction of unreferenced values"+

[nit] This talks about the fraction of values but we're really talking about the fraction in terms of bytes (btw the TargetGarbageRatio description is similar). Maybe
"fraction of space wasted by unreferenced values which triggers blob-file rewrite compactions (1.0 disables these compactions)."

---

pkg/storage/pebble.go line 388 at r2 (raw file):

		"storage.value_separation.compaction_garbage_threshold",
		"the max garbage threshold configures the maximum fraction of unreferenced values"+
			" that may be garbage without performing blob-file rewrite compactions. When"+

[nit] no space between When and unreferenced. I'd standardize the space at the end everywhere

---

pkg/storage/pebble.go line 392 at r2 (raw file):

			" triggered to reclaim disk space.",
		1.0, /* default; disables blob-file rewrites */
		settings.FloatInRange(0.0, 1.0),

[nit] Maybe set 0.1 as the minimum

[jbowens]

TFTRS!

Reviewable status: complete! 0 of 0 LGTMs obtained (and 2 stale) (waiting on @annrpom)

[jbowens]

bors r+

[craig]

Build succeeded:

• linux_amd64_fips_build
• local_roachtest_fips
• check_generated_code
• docker_image_amd64
• local_roachtest
• lint
• unit_tests
• examples_orms
• acceptance
• linux_amd64_build

[jbowens]

blathers backport release-25.3