release-22.2: allocator: expose LeaseRebalanceThreshold as a cluster setting

[wenyihu6]

Backport 1/1 commits from #105924.

/cc @cockroachdb/release

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This patch adds a new cluster setting --
kv.allocator.lease_rebalance_threshold. This setting controls the minimum
fraction away from the mean a store's lease count can be before it is considered
for lease-transfers. The default setting is 0.05.

Fixes: #105909

Release Note (allocator): cluster setting
kv.allocator.lease_rebalance_threshold can now be used to control the minimum
fraction away from the mean a store's lease count before it is considered for
lease-transfers. The default setting is 0.05.

---

Release justification: low risk to the existing functionality

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is 

[wenyihu6]

Same comment from the previous PR - non-trivial backport to the 22.2 branch due to the absence of the cluster setting field on the allocator. Happy to close this PR if the change is considered substantial.

Would appreciate a second pair of eyes on adding the cluster setting field to the allocator. Most of the changes were made by just observing the existing pattern observed on the master branch.

[kvoli]

Are you going to squash the first commit into the second before merging?

Reviewed 6 of 6 files at r1, 2 of 2 files at r2, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @wenyihu6)

[wenyihu6]

Are you going to squash the first commit into the second before merging?

Reviewed 6 of 6 files at r1, 2 of 2 files at r2, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @wenyihu6)

Yeah. Just squashed them.