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: #16780

Checklist

• [x] "Backport to:" labels have been added if this change should be back-ported to release branches
• [x] If this change is to be back-ported to previous releases, a justification is included in the PR description
• [x] Tests were added or are not required
• [x] Did the new or modified tests pass consistently locally and on CI?
• [x] Documentation was added or is not required