release-24.3: workload/schemachanger: limit size of datums during insert

[blathers-crl]

Backport 1/1 commits from #139736 on behalf of @fqazi.

/cc @cockroachdb/release

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Previously, the insert statement in the randomized schema changer workload could have unbounded sizes for individual datums. This patch adds size limit for datums in the insert operation, where datums will be regenerated a limited number of times until they are below a maximum size.

Fixes: #139494

Release note: None

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Release justification: test only change

[blathers-crl]

Thanks for opening a backport.

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[cockroach-teamcity]

This change is