VDiff: Improve row diff handling in report

[mattlord]

Description

The sample row diffs in the vdiff show --verbose --format=json report — when row differences were found between the source and target — were too often unhelpful. The reasons being:

1. The column values were truncated pretty aggressively, which often meant that you could not see the actual differences in values between the source and target
2. The PK values were also truncated (when not using the --only-pks flag), meaning that you could not examine the row manually on the source and target to determine the difference(s)
3. You could not affect the truncation behavior

This PR addresses these issues as follows:

• We add a vdiff create flag called --row-diff-column-truncate-at which allows you to control the truncation behavior (defaults to 128 chars and you can specify a value < 1 to avoid any truncation)
  • This is a create flag rather than a show flag because we build the row diffs as we perform the actual diff of the given table, storing the results as we go along in the sidecar DB's _vt.vdiff_table table
• We always include the PK columns first in the row output, and we never truncate those columns
  • So this part is now the same with and without --only-pks

Related Issue(s)

• Fixes: Feature Request: Add VDiff command flag for controlling row content in diff report #16527

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation: Documentation for https://github.com/vitessio/vitess/pull/16618 website#1831

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 21.95122% with 32 lines in your changes missing coverage. Please review.

Project coverage is 68.98%. Comparing base (538dd4c) to head (a9117da).
Report is 16 commits behind head on main.

Files	Patch %	Lines
go/vt/vttablet/tabletmanager/vdiff/report.go	0.00%	20 Missing ⚠️
go/vt/vttablet/tabletmanager/vdiff/table_differ.go	0.00%	10 Missing ⚠️
...d/vtctldclient/command/vreplication/vdiff/vdiff.go	50.00%	1 Missing ⚠️
...vt/vttablet/tabletmanager/vdiff/workflow_differ.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16618      +/-   ##
==========================================
- Coverage   68.98%   68.98%   -0.01%     
==========================================
  Files        1562     1562              
  Lines      200690   200692       +2     
==========================================
- Hits       138449   138443       -6     
- Misses      62241    62249       +8     

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