feat: add option for datagen to produce indefinitely (MINOR)

[vcrfxia]

Description

BREAKING CHANGE: If no value is passed for the KSQL datagen option iterations, datagen will now produce indefinitely, rather than terminating after a default of 1,000,000 rows.

This change is valuable because previously there was no way to specify that datagen should produce indefinitely. This PR adds this ability, and updates it to be the default behavior. (Users can still use the old behavior of terminating the process after a fixed number of rows by passing that number via iterations.)

Testing done

Manual.

Reviewer checklist

• Ensure docs are updated if necessary. (eg. if a user visible feature is being added or changed).
• Ensure relevant issues are linked (description should include text like "Fixes #")

[big-andy-coates]

Thanks @vcrfxia

Might it not be better to just allow the app to be run without the iterations flag if you want it to run for ever?

[vcrfxia]

@big-andy-coates good point. The current behavior is to default to producing 1,000,000 rows if the iterations property is not explicitly set. Unless we're concerned with maintaining backwards compatibility of this default, I will update the PR to change the default behavior to be indefinite production, removing the need for the special -1 value in the process.