For simultaneous multi-stat composites switch from `join` to `combine`

Currently when providing multiple stats to `summarise` for compositing. We run them individually and then join together with `inner_join.tidyee` which is wrapped inside of `purrr::reduce` for handling >2 joins.

I wonder if eventually it would make sense eventually to switch to using `rgee::ee$ImageCollection$combine()` .

Don't think this is urgent, but worth thinking about the pros and cons