Extend First/Last values for Step Interpolation

[hulien22]

From @StavisT.
For step interpolation, there is sometimes a desire to extend the first and last datapoints to the end of the window. This can be easily implemented by checking for a stepInterpolation call and then duplicating the first/last endpoints received and setting their times to the beginning/end of the query range.

The only concern is to figure out when this should happen: whether we should do it for every step interpolation call, or if there should be a separate option for this (that would allow it for any type of call). I'm against the first option as it might be confusing to users since this would be the only aggregate type that does this. If we go with the latter, then we need to figure out where to put this option.