VReplication: Relax restrictions on Cancel and ReverseTraffic when writes not involved

[mattlord]

Description

This PR makes two related ease-of-use improvements to VReplication workflows and in particular multi-tenant MoveTables workflows:

1. It allows you to Cancel/Delete a workflow if you've only switched read traffic (tablet types of replica,rdonly). Previously you would have to first do a ReverseTraffic step before you could Cancel/Delete the workflow. This was generally a minor pain, but it was a larger pain for multi-tenant MoveTables workflows as you could not reverse traffic at all for them. So if you had switched reads you were stuck and had to perform manual work to get things in the desired state.
2. It allows you to do a ReverseTraffic for reads only in a multi-tenant workflow. This allows you to revert that partial switch without having to again perform some manual work to get things in the desired state.

Related Issue(s)

• Fixes: Feature Request: MoveTables allow Cancel when reads have been switched, but writes not #17127

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

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[codecov]

Codecov Report

Attention: Patch coverage is 76.31579% with 9 lines in your changes missing coverage. Please review.

Project coverage is 67.44%. Comparing base (a9bab82) to head (d5ebfc9).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtctl/workflow/server.go	75.67%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17128      +/-   ##
==========================================
+ Coverage   67.42%   67.44%   +0.01%     
==========================================
  Files        1569     1569              
  Lines      252120   252127       +7     
==========================================
+ Hits       169997   170042      +45     
+ Misses      82123    82085      -38     

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[deepthi]

Good tests!