Fix onlineddl change col json

[shlomi-noach]

Description

Resubmission of #14093, with an additional onlineddl_vrepl_suite test. Creating this PR as I do not have push access to hubspot repo.

Related Issue(s)

#14089

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[shlomi-noach]

#14093 is good to go and this PR is not required.