Support `.groupby(['foo', 'bar'])` as sugar for `.groupby(foo=UniqueGrouper(), bar=UniqueGrouper())`

[dcherian]

Once #9372 goes in we will support grouping by multiple variables.

@max-sixty makes a good point re: ergonomics:

I missed previous discussions / can decide this later — but do we want to have da.groupby(['foo', 'bar']) as sugar for da.groupby(foo=UniqueGrouper(), bar=UniqueGrouper())?

Originally posted by @max-sixty in #9372 (comment)

I think this would be great. da.groupby(foo=UniqueGrouper(), bar=UniqueGrouper()) is a bit too much typing for a very simple expression.

[dcherian]

@max-sixty are you able to do this?

[max-sixty]

I can try to have a look at the weekend

(I realize I'm in your debt but generally when I commit to things, I end up not doing them, so I won't commit here. But would like to reciprocate some of your recent work and agree it makes sense for me to take it)

[dcherian]

no pressure! ;)