cache: add k8s-aligned ringbuffer helpers

[yedou37]

Add Len, ReplaceLatest, RemoveLess, and ensureCapacity functions to ringBuffer, and cover them with unit tests.

These helpers are not used in etcd today, but they keep the ringBuffer implementation aligned with the k8s snapshot ring work discussed at: kubernetes/kubernetes#138338 (comment)

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: yedou37
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[k8s-ci-robot]

Hi @yedou37. Thanks for your PR.

I'm waiting for a etcd-io member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

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[Jefftree]

/ok-to-test

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.33%. Comparing base (e775342) to head (6451452).
⚠️ Report is 78 commits behind head on main.

Additional details and impacted files

see 79 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #21676      +/-   ##
==========================================
+ Coverage   68.48%   70.33%   +1.85%     
==========================================
  Files         432      426       -6     
  Lines       35540    35215     -325     
==========================================
+ Hits        24339    24768     +429     
+ Misses       9789     9056     -733     
+ Partials     1412     1391      -21     

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