VTAdmin(API): Add workflow start/stop endpoint

[beingnoble03]

Description

This PR adds start and stop VReplication workflow endpoints in the VTAdmin-API. These are to be further used in VTAdmin-Web.

Related Issue(s)

• VTAdmin: Enhance VReplication functionality in vtadmin UI #11690

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 was added or is not required

Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 3.12500% with 62 lines in your changes missing coverage. Please review.

Project coverage is 68.90%. Comparing base (a54d5ec) to head (da45f9e).
Report is 5 commits behind head on main.

Files	Patch %	Lines
go/vt/vtadmin/api.go	4.16%	46 Missing ⚠️
go/vt/vtadmin/http/workflows.go	0.00%	16 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16658      +/-   ##
==========================================
- Coverage   68.93%   68.90%   -0.04%     
==========================================
  Files        1562     1562              
  Lines      200767   200831      +64     
==========================================
- Hits       138407   138376      -31     
- Misses      62360    62455      +95     

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[mattlord]

Nice work, @beingnoble03 ! I only had one small request about the span name. If you can change that — unless I'm missing something and you or @rohit-nayak-ps disagree — then I'll go ahead and merge it.

Thanks!

[mattlord]

We should use the API call name for the trace span here and for stop as well. WorkflowUpdate has its own trace span. Then the action for the span is also obvious (now it's not clear what we're updating in the span info).

[beingnoble03]

Oh! Thanks for noticing. I forgot to change here.