VReplication Workflow command: Allow stop/start on specific shards

[rohit-nayak-ps]

Description

While importing from massive external databases into a sharded Vitess cluster with multiple shards, the initial load from multiple copy phases is significant. Right now we don't have the ability to stop or start a workflow on a subset of shards. Allowing this feature will allow us to

• create a workflow with auto-start=false
• run workflows on limited number of shards to reduce load on the external database

This PR adds the ability to stop/start shards by passing the --shards flag.

Note

We were already accepting --shards but it was being ignored.

Related issues

• Fixes: Allow stopping and starting specific shards for VReplication workflows #17584

Checklist

• "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

Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 67.68%. Comparing base (99856a2) to head (3b83410).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...tctldclient/command/vreplication/workflow/state.go	0.00%	4 Missing ⚠️
go/vt/vtctl/workflow/server.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17581      +/-   ##
==========================================
- Coverage   67.69%   67.68%   -0.01%     
==========================================
  Files        1586     1586              
  Lines      255403   255409       +6     
==========================================
- Hits       172883   172875       -8     
- Misses      82520    82534      +14     

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