feat: Add key cardinality testing feature

[kentquirk]

Which problem is this PR solving?

• We've seen issues where the key cardinality of things being fed to a sampler is longer than the sampler AdjustmentInterval. This gives loadgen the ability to generate fields that will only include all possible values once the generator's sample period has completed.

So mykey=/k50,60 will generate a field called mykey that will have values of cardinality 50, but not all possible values will be used until 60 seconds have passed.

Includes testing and benchmark code, as well as a readme update.