[feat] Add option for disabling fragmentation

[jdheyburn]

Happy to rename the option to something more suitable. Status quo should be the default (fragment if parallelism > 1). User can disable by setting disableFragmentation: true in their spec.

Closes #90

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[yorugac]

Closing this PR: it's not going to be merged and it's been awhile 😅 The related issue remains open: if someone would like to argue for adding a similar logic to k6-operator, please comment and vote in the issue!