Feature: Add ability to list Fields in all methods (i.e. mean(), min(), max()) and `as()` method

Historically, it has been a challenge for our Kapacitor users to downsample a lot of Fields assuming they desire any of the following:
* Fields with their original names (new RP in most cases, so this is fine)
* Don't want to hardcode a Task for every single Field they have
* Don't require templates (templates require even more upfront work anyways)

Having the ability to simply list the Fields you want to aggregate and what you want to name them in a single Task could potentially reduce the amount of TICKscript needed by multiple orders of magnitude.

Related EAR: https://github.com/influxdata/EAR/issues/2218