v20 backport: CI upgrade/downgrade tests for Online DDL / throttler / vreplication flow

[shlomi-noach]

This is the backport of #16017

Backporting manually as I forgot to add the backport label.

Description

Adding upgrade/downgrade tests as follows:

• The tests will run an X version VTTablet primary, with Y VTTablet replica. See breakdown below.
• Enable tablet throttler.
• Create and populate some table.
• Create a load. The load takes throttling into account so that it does not overwhelm the cluster.
• Run Online DDL with vitess strategy, thereby running VReplication that is also using the throttler.
• Throttle and unthrottle the migration.
• Validate migration behavior while under load. Complete the migration.
• Expect successful completion of the migration, with cut-over as graceful as possible (but will resort to forced cut-over if unable to complete in a timely fashion).

See more details in https://github.com/vitessio/vitess/pull/16017/files#diff-dc6e3889488266071984d6da1f44795b7b06bf5ff6504c8e9aa359598918230aR17-R37

This will test cross version compatibility of the throttler, of vreplication, of Online DDL. It runs onlineddl_flow in the following setups:

• Primary at version N (current) and replica at N-1
• Primary at version N-1 and replica at N
• Primary at version N+1 and replica at N
• Primary at version N and replica at N+1

Each such flow runs at about 1 minute, which is why I chose to aggregate them all in the same workflow test (as opposed to splitting to "new" and "old")

EDIT: sample execution flow: https://github.com/vitessio/vitess/actions/runs/9348746367/job/25728525330?pr=16017

Related Issue(s)

No specific issue, but with #15988 in mind.

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

All modified and coverable lines are covered by tests ✅

Project coverage is 68.22%. Comparing base (83c14fa) to head (e0970b6).

Additional details and impacted files

@@               Coverage Diff                @@
##           release-20.0   #16065      +/-   ##
================================================
+ Coverage         68.20%   68.22%   +0.02%     
================================================
  Files              1541     1541              
  Lines            197330   197330              
================================================
+ Hits             134591   134635      +44     
+ Misses            62739    62695      -44     

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