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ARROW-17362: [R] Implement dplyr::across() inside summarise() (#14042)

This PR implements `across()` within `summarise()`.  It also adds the `.groups` argument explicitly to the `summarise()` function signature instead of being passed in via `...` (this was necessary to prevent test failures after the addition of `expand_across()` to `summarise()`).

Authored-by: Nic Crane <thisisnic@gmail.com>
Signed-off-by: Nic Crane <thisisnic@gmail.com>