NIFI-12410 Refactored to allow for 'Starting Row' to have flowfile attributes scope.

[dan-s1]

Summary

NIFI-12410
Refactored the 'Starting Row' to have flowfile attributes scope and changed the validator on this property from NON_NEGATIVE_INTEGER_VALIDATOR to POSITIVE_INTEGER_VALIDATOR to prevent users from entering 0.

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[exceptionfactory]

Thanks for the improvement @dan-s1, the changes look good! +1 merging