Add lazy `aggregated_by`

[zklaus]

At the moment, collapsed is the only lazy statistics method. It would be useful to also have aggregated_by in a lazy version, i.e. allow for the use of lazy aggregators.

This touches on #2418, and #2928.

[valeriupredoi]

any chance collapsed with kwargs alike weights can be converted to lazy as well? There are really large differences in performance between collapsing with/without weights, see https://github.com/ESMValGroup/ESMValTool/issues/887 (my apologies if this comment is slightly off-topic)

[zklaus]

@valeriupredoi I can definitely have a look. But first this needs to be finished.

[valeriupredoi]

absolutely - I will actually have a talk with @bjlittle about this (and other iris stuffs) on Monday 🍺