Aggregations: Adds a new GapPolicy - NONE

[colings86]

This is to allow the skip gap policy to always skip execution if values are missing. The none gap policy will set the value of the missing metric to NaN. In practice the skip and non gap policies work the same in all pipeline aggregators except the bucket_script agg, where skip will not execute the script for buckets where any of the input metrics are missing, and none will pass Double.NaN into the skip as the value of these missing metrics

[jpountz]

I'm not too happy with having both none and skip given how close they are. I think we need to pick one.

[clintongormley]

@jpountz If we have to choose one then it should be skip, as this will be the most common use case for scripting. However, this will prevent users of bucket_scripts from processing buckets with truly null values (distinct from zeroes).

We don't know yet whether this second use case is valid or not. I'd suggest leaving this PR as stalled for now, and we can always revisit it if users need it.

[colings86]

@clintongormley we cannot leave this PR as stalled as skip does not work as we intend it to for the bucket_script aggregation currently (it passes NaN into the script rather than skipping execution). Would you rather that I change this PR to just make bucket_script not execute the script if any of the variables are missing in the bucket?

[colings86]

Skip gap policy has been fixed in #11970. Closing this for now as we need to think about whether other gap policies are required