xds/clusterresolver: [alternative] Revise configbuilder childname generator to fix leakage issues with merge and split scenarios

[erenboz]

Alternative implementation for #8531

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.03%. Comparing base (5ed7cf6) to head (cec1bc6).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8532      +/-   ##
==========================================
+ Coverage   81.99%   82.03%   +0.04%     
==========================================
  Files         412      412              
  Lines       40520    40450      -70     
==========================================
- Hits        33225    33185      -40     
+ Misses       5904     5887      -17     
+ Partials     1391     1378      -13     

Files with missing lines	Coverage Δ
...alancer/clusterresolver/configbuilder_childname.go	100.00% <100.00%> (ø)

... and 21 files with indirect coverage changes

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