VReplication SwitchTraffic: for a dry run check if sequences need to be updated

[rohit-nayak-ps]

Description

When resharding an unsharded keyspace, sequences need to replace autoincrements. These sequences need to be set to an initial value which is above the max id. Doing this for each table is tedious and the --initial-target-sequences option was added to SwitchTraffic for this purpose.

However ff there are a lot of tables being resharded the process of getting the max ids from all shards and then updating the value on the unsharded keyspace can take too long. So it might still make sense for the users to update the sequence manually.

This PR adds the ability to find the sequences which need to be updated as part of switching traffic. The user can then decide on setting a value on the targets to a safe value and then not use the --initial-target-sequences flag.

The code to get the max values was intertwined with the rest of the sequence setting code. So this PR also refactors the existing code to have a single code path for the dry run and regular path.

Related Issue(s)

fixes #17831

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Documentation was added or is not required

Deployment Notes

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[codecov]

Codecov Report

Attention: Patch coverage is 68.04734% with 162 lines in your changes missing coverage. Please review.

Project coverage is 67.46%. Comparing base (2118bc3) to head (2f53594).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtctl/workflow/sequences.go	68.22%	156 Missing ⚠️
go/vt/vtctl/workflow/switcher_dry_run.go	62.50%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17842      +/-   ##
==========================================
+ Coverage   67.45%   67.46%   +0.01%     
==========================================
  Files        1592     1593       +1     
  Lines      258167   258271     +104     
==========================================
+ Hits       174143   174253     +110     
+ Misses      84024    84018       -6     

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