Add extraction of joinkeys from nested structures

[colmsnowplow]

If a joinkey is in a nested field (eg. the page_view id in an array), modeling the data around performance can be a challenge.

I suggest we add some way of indicating a joinkey in a custom event, and extract those values into their own column alongside loading the data as a nested object.

(A 'joinkey' option in the schema feels most straightforward).

Have spoken about it before, adding as an issue for future reference.

[alexanderdean]

Really neat idea!