vdiff: factor Summarize out from vtctldclient

[mdlayher]

Description

Factor out buildSingleSummary as vdiff.Summarize, which generates a summary of the VDiff operation based on the VDiffShow RPC response.

Related Issue(s)

• Fixes: Feature Request: export vtctldclient VDiffShow summarization code #16780

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• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

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[codecov]

Codecov Report

Attention: Patch coverage is 75.57252% with 32 lines in your changes missing coverage. Please review.

Project coverage is 68.95%. Comparing base (155ede0) to head (a77b4af).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vttablet/tabletmanager/vdiff/summary.go	75.38%	32 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16781      +/-   ##
==========================================
+ Coverage   68.93%   68.95%   +0.02%     
==========================================
  Files        1565     1566       +1     
  Lines      201764   201764              
==========================================
+ Hits       139088   139129      +41     
+ Misses      62676    62635      -41     

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[mdlayher]

Small bit of trailing whitespace here and on line 420. I suspect it's not load bearing but if it turns out to be, I can undo this.

[mattlord]

@mdlayher I like this idea! Makes total sense to me. I don't, however, think that tabletmanager is the right place for this code as it's really VtctldServer code that's working with vtctldata protos (rather than TabletManager code working with tabletmanagerdata protos), which primarily lives in the workflow package (go/vt/vtctl/workflow). I think we should probably create a go/vt/vtctl/workflow/vdiff.go file and move this code there along with the other VDiff functions that are currently in workflow/server.go — we need to start splitting that file up anyway as it's getting massive. That make sense?